88 Caprice Hesitation / Stalling

My mint cond 88 Caprice with 55,000 miles has been a pain in the butt for almost a year now. The car starts and runs fine cold. Once warmed up (5 min), it hesitates everytime you pull away. Sometimes a minor hestitation, sometimes a noticable stumble. When warmed up, it often idles rough, and on a few occasions, has stalled in traffic when no throttle was depressed. (coming to a stop)

Two carbs, one new and one 2nd hand carb were installed, no improvement. New computer installed, all vacuum hoses replaced, EGR tested, 02 sensor replaced, throttle position sensor disconnected/reconnected, mixture solenoid disconnected, plugs and wires replaced, new distributor, AIR injection disconnected / reconnected, etc etc. Everything has been attempted and nothing makes any improvement. It runs fine cold, so I've ruled out a fuel pump or cam lobes problem or exhaust restriction.

So I gave up, installed a carb off an 83 caprice (with no computer controls), produces error codes for mix solenoid and TPS (is none), but car idles fine, but still hesitates on acceleration! But on a recent trip I noticed at sea level, the hesitation became negligible. When i returned home (4000ft elevation), the hesitation got noticable worse again. So... Could this be a MAP sensor or BARO sensor? I'm running out of options. Appreciate any thoughts people have on this. Very frustrating car to work on compared to my 79 and 86 caprice which have been problem free. Thanks. Happy motoring.

I would look for a loose or broken vacuum line somewhere - check all of the A/C lines as well (vacuum control lines) sometimes this can cause problems.

Timing?

Let me know if you find the answer to this. My '89 does the exact same thing sometimes. Most of the time it's fine, but sometimes it feels like the engine is missing. It idles rough alot, but hasn't stalled yet.

My car has EFI, so I guess the fuel delivery system can be ruled out...

This being a late reponse I'll hope you get it.I've had GMs for many years and found one major thing that is passed up when trouble shooting carbed engines,the heat oven and piping going to the air breather,if this is deteriated and or missing you'll have the problem your having since your engine relies on warm air for its mixture and it will show when the choke pulloff has opened the choke.Also make sure the vacuum line is in good shape.If the system is working correctly just after starting the engine the snorkle of the air cleaner housing should start to heat up.