I have a 2000 Nissan Xterra and that will begin to run very rough when stopping and stall at red lights. I can prevent it from stalling if I put it in neutral but then need to rev it up to 1K RPM and put into gear to get going again. The garage initially told me that the transmission needed rebuilt to the tune of $2K, however, when I dropped it off to have the work done they re-evaluated and determined that the transmission is fine but they have no idea why it is stalling…? It will be fine for a few days or even weeks, then starts acting up again… Any thoughts?
How many miles are on it? What is the state of maintenance? spark plugs? wires? fuel & air filters? has anyone checked the fuel pressure? Has the check engine light been on? Do the mechanic(s) ever mention anything about scanning the computer and if so, what - in particular - was found? Do you have any error codes recorded on any of the paperwork from them? (These would look like “P0123”).
Without knowing about all of that one thing that would fit would be a torque converter clutch that is stuck on. That’s likely where the idea of rebuilding the transmission is from. Is the “garage” actually a transmission shop? While good general mechanics can do most of the work on most any car, transmission knowledge and work is pretty specialized. So I’d ask around and find the best local transmission shop you can and take it there.
It has about 160,000 miles; I have not changed the spark plugs/wires since I purchased about 120k miles ago (not ideal, i realize); air filter is good; fuel filter hasn’t been changed since about 100k miles; no one has checked the fuel pressure; no check engine lights; no codes when they scanned.
The garage I took it to handed it over to a transmission specialty shop who, in turn, diagnosed the rebuild based on the idea that the torque converter was sticking. However, they took the pan off and said that everything was “clean as could be” and refused to do the rebuild because they no longer thought that the torque converter was the problem. Based on the age/mileage of the truck, I’m trying to avoid replacing part after part trying to fix the problem without knowing what is causing it… very confused.
Ok - start by separating two very different things that you might ask mechanics to do (or that you might figure out how to do yourself). One thing is maintenance. Just on mileage and time you should absolutely get a new fuel filter and at least have someone pull and check the plugs. Odds are that you need new plugs too. These things are not done to “repair” problems but are normal things that need to be done as a matter of course just to properly maintain a piece of machinery.
The second kind of thing is “repair” - as in when something breaks - like maybe some part of your TCC lock up system gets out of whack.
Anyway, when your car is in gear the engine is under more load then when it is in park and neutral. If the car is only marginally running to begin with, putting it in gear can make it run rough or stall just b/c of simple maintenance things - like a weak spark or low fuel pressure from a clogged fuel filter. So you really want to make sure none of that is going on, but more to the point you want to do that even if it doesn’t “fix” your problem. These maintenance items are not actually “fixes” (repairs) - they’re just maintenance.
Anyway, if all of that gets to be in order you should make sure that you don’t have any vacuum leaks, that your idle air control valve is cleaned and checked along with your throttle body. You might also get some specifics from the transmission shop about how they reached their conclusions.
I am NOT a mechanic whatsoever. But the problem does not sound like a tranny problem to me at all. As was pointed out, I think the car idles rough and stalls because of a maintenance backlog.If filter are clogged and spark plugs get old, I believe a rough idle is very common. And, of course, revving it up will prevent it from doing so. That you have to put it in neutral to rev it up is most likely only a coincidence. It would also not stall if you revved it up in drive and went - you’d just run the red light 
I had a similar problem recently and it turned out to be a cracked plastic boot in the air intake. The car wouldn’t get enough air and stall.
Take it with a grain of salt as I am not a mechanic but if I read that spark plugs have last been changed 120K miles ago I suspect some sort of maintenance backlog. Remedies were profoundly laid out by cigroller above.