It seems like you are doing all the right stuff. Your compression test results should give you a lot of hope you’ll soon have a smooth running machine there. I’m assuming you are checking for diagnostic codes from the computer system and not finding any, right? I can see why you want to avoid the 100 pound expense for new wires, given they appear from your testing to be ok. Maybe as a compromise just remove and clean them in soap and water, then inspect the insulation carefully on the bench under good lighting. I found a problem on one the my truck’s HV wires doing that. I had a clue though, a noticeable misfire, and I heard a slight zap sound now and then. A small crack and signs of arc’ing was visible on that wire as viewed on the test bench , but barely. I was able to repair the trouble spot using some HV electrical tape rather than replacing the wire, which solved the zapping and the resultant misfire.
My old Rabbit had the Bosch K-Jetronic fuel injection. Very sensitive to fuel quality. For comparison, I’ve never had a fuel injection problem on my Corolla in 25 years, but on the Rabbit I had to fix something w/ that every 6-9 months after the car aged to the 6 year mark. When that system clogged up – due to grit in the fuel – the symptom was lack of power. I’d press on the accel pedal at 25 mph, press it all the way to the floor, maybe I’d increase speed to 26 mph … lol … When that happened I had to disassemble the fuel distributor into its various parts and clean them all up to get it to run well again. I never had a problem with the injectors clogging, but I did have to replace the injector o-rings a few times. The L-Jetronic is a different beast though, but if you can find instructions on how to disassemble & clean it out, that’s probably something to try. On the Rabbit is was very easy to clog the system as part of a fuel filter change ironically. Somehow dirty fuel from the old filter would get past during the change-out.
When I read your comment
“The car runs well for a minute or so, then will run poorly for a min or so. When running poorly, there is a lack in power, fluctuating idle, splutter when reving (sometimes with backfire), occasional stalling.”
my first thought is a problem with the ignition system that generates the spark. That’s often heat sensitive and intermittent. On my Rabbit it was just a points and single coil system. The only problem I ever had was a cracked coil that made it stall if I ran over a puddle of water. The crack was on the underside and so tiny it was nearly impossible to see. For the Rabbit, it was a big coil on the firewall, not the small coils more common now that are located inside the distributor. If you have a big coil like that, remove it, clean it up, and inspect it on the bench under good lighting & using a magnifying glass for really tiny cracks. If you have a coil and ignition module inside the distributor, which senses the distributor shaft turning to synch up with the crankshaft, that could be the problem.
I should add that o’scope testing of the ignition system is very helpful to diagnose an ignition system problem. That’s what I’d do first if I had the same symptoms with one of my cars. If you lack the equipment or knowledge how to do it, at some point your best bet may be to have to ask a shop do an o’scope analysis.
Edit: One more idea, do an EGR test. You may have an EGR valve that is sticking open.