Concur w/db above, 110-130 psi compression could well explain the misfire symptoms you are having. If you are certain compression isn’t the problem however, if I had that problem I’d do what is called an injector balance test. How that is done varies car to car, but the idea is you pulse the injectors one by one a metered amount, while monitoring the fuel rail pressure. If each injector is the same, the fuel rail pressure will drop the same amount. If one or two don’t drop the fuel rail pressure as much as the other two, that means those injectors are injecting less gas for a given pulsing sequence, which will definitely make the engine perform poorly.
At the moment, I’m working on a truck that has multiple misfires
And it’s DEFINITELY due to low compression, far lower than normal specs
I did a cylinder leakdown test, and it has severe leakage, clearly heard air coming from the throttle body
I peered into the cylinder with my videoscope, and saw EXTREMELY pitted intake valve seats. Gee, I wonder why the compression is low, and I wonder why the leakage is 40% 
The engine needs a complete valve job
BTW . . . that engine had FAR higher compression than OP’s engine
Perhaps OP’s main problem is that tired old engine, but he remains convinced that is not the case
Has anyone besides the OP seen a 6 run on only 2 cylinders? I haven’t.
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That’s an interesting question. I expect OP’s comment is a bit of hyperbole. Maybe 4 of the cylinders are occasionally misfiring is all. but it would be an interesting experiment to start with a properly running 6 and remove wires one by one, see if there’s a way to keep the engine running with only 2 wires hooked up.
No they are not misfiring, they are not working at all. You can remove all 4 middle spark plug wires and it runs 100% the same way. When you remove either the 1st or last plug wire it stalls. By the way it idles perfect like this. Purs like a kitten at idle.
up on this thread it was a good idea IMHO: “injector filters are clogged”
if you say that idle is ok, but under load it dies immediately, that might be the case
real idling on 2 cylinders would be very erratic, but you say “it purs” => further plays into the theory of restricted flow through injectors
I’ve recently cleaned my injectors quite crude way and it costed me only a little bit of time to solder wiring and a can of carburator cleaner:
- remove all injectors, clean with carb cleaner from external dirt
- make some wires to securely connect to injector, have battery to energize them (short taps only!)
- fill injector with carb cleaner, get blow-gun with rubber tip to the inlet, aply pressure
- shortly tap wires to battery to open flow
- repeat
in my case, 4 injectors were in perfect shape, one had slight restriction, one had something like half of nozzles to squirt gas to the side, quite restricted flow
after 3-4 runs of whatever small amount of carb-cleaner I could get to injector in one time, all were squirting evently and in good pattern
after I’ve put injectors back on the car, idle became smoother, but, importantly, power under high load improved a lot
if your injectors are heavily restricted, you can get rebuild kit off ebay for under $20: it will contain both o-rings and replacement filters… besides, replacing o-rings is likely a must for your car’ age
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So this car has had power problems for at least a year and a half. Your compression sucks, and it seems to be running on 2 cylinders. In my opinion you need to seek competent, professional help to get past the denial and on to acceptance that this engine is shot and move on. Sorry just had to say that.
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If you say it idles ok on only 2 cylinders, I have no reason to disbelieve you. Just surprising is all it could idle with a purr with only 2 cylinders. But there’s many small 4 cycle gas engines like for lawnmowers than idle smoothly with only 1 cylinder, so such a thing may indeed be possible. Lawnmower engines don’t have 4 dead cylinders they have to compress the intake air in them each stroke tho. That seems like it would create quite a load on the working cylinders. But maybe at idle not much air mass gets in, so not much air needs to be compressed.
The other puzzling thing, you’d think if 4 cylinders of 6 were not firing at all, you’d get huge backfires from all the gas and air passing directly through the engine & into the exhaust system. Perhaps the problem is what @andriy.fomenko is saying above, four totally clogged injectors. That would explain the lack of backfires. Well, at least you got an interesting problem to work on there OP! … 
Thx once it warms up enough I will work on checking the injectors. If I must I have another engine I could drop in it. It’s out of the same car same year. It ran good when I parked it. It had a oil leak I think out of the back seal. At the time I did not want to screw with fixing it so I just parked it. Now that it’s pulled out that would be easy to fix. I pulled it just before it got cold. It has been sitting for a while though so no way to know how well that will run. I’d rather not have to go that way.
I read an article in a car-restoration magazine recently, forget which one it was, where they built an engine test fixture, didn’t cost that much. Basically it is just an engine stand that holds a radiator/radiator fan, battery, ecm, and a fuel tank is all. Plus some gadgets that allow you to vary the throttle. So with that you can run the engine out of the car. If you are the scientific type it would be a good way to test the replacement engine to make sure it is working before you bother install it. I seem to recall the vendor for the equipment is Summit Racing.
I have seen 2 engine test stands that were built in house by local shops. One had an in house built dynomometer on it.
And 2 cylinder engines have significantly heavier flywheels than 6 cylinders. More weight is necessary to carry the crank from one power stroke to the next. A V-6 idling ‘normally’ with 4 cylinders dead seems impossible.
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This is a straight 6 if that matters anything.
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Well, a 1-5-3-6-2-4 firing order will have 1 and 6 firing evenly (smoothly).
Have you tried puffing a little ether (starting fluid) into the intake while it’s idling?
I do not think I have. I could try that Or what if also I tried to dump some fuel into each dead cylinder and replace the plugs and see what it does.
fyi, here’s the link to the article about the engine stand used to test running engines, I was mentioning above.
Fantastic car. Glad to hear you are keeping it alive!
Maybe you missed it. The OP is not keeping anything alive he is merely keeping it around.
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Of course it is . Any 6 cylinder that idle smoothly on 2 cylinders is a fantastic vehicle.
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I never thought of it that way VOLVO_V70. A couple summers ago it did run better and you could feel it has potential for a crap load of power at that time. Since it did run better a year before and nothing was really done to it that would have made it run worse, it seems very likely it could be the injectors got more clogged over a years time. The quake mechanic I took it to said he tested the injectors but it looks like they were never touched. By the way this quake did not cost me anything. he wanted storage fees for having it sit out outside for a few months, which he did nothing to it. I gave him hell and told him if he is going to charge me anything I would take him to court. Plus I do not think he is legal, so I threaten him with problems with the law and with the tax man. As if he is not legal he is not paying taxes, which they frown on that. Not that I in any way want to get our communist IRS and state rev people anymore money. But I got it back and it costed me exactly $0.