My 92 Bonneville stalls when breaking to a stop. Sometimes it stumbles but
doesn't stall. Rarely but it may stumble or even stall when driving with my
foot off the accelerator for maybe 15 or 30 seconds. Works OK when driving
and if I accelerate hard it doesn't seem to hesitate and continues to run OK
as I accelerate normally. It runs somewhat rough at idling.
I replaced the PCM with a working one I have and it did the same thing.
Any help would be appreciated.

Tony

Could be a number of things.

Vacuum leak, check all your lines, spray carb cleaner on them, the RPM will change if you spray a leak.

Bad TPS https://www.gmforum.com/t279319/

Try doing a seafoam on the car. 1/3 can in a vacuum line near the throttle body, the other 2/3 can in the fuel tank. Change oil shortly after doing this. I've seen this do great things.

I have not encountered this yet in my Bonny, but I had a similar experience in my ZX3. It was a combination of a low idle (100 rpms down due to a Throttle Position Sensor set screw that came loose) and a vacuum leak. I lean towards a vacuum issue like Dan mentioned, which is easier to sort out than what else might be happening. Keep us appraised of your progress.

It'* really interesting..when I read a post usually a couple things pop into my head.. I've been watching too much House lately I supposed.

Check the ole fuel pressure and then note if the car usually stalls when under 1/2 tank and maybe if you are heading downhill or on a turn. 92/93 fuel tanks had an issue with baffles breaking loose, that would allow the fuel to all slosh forward during a stop and starve the engine.

If you add those couple things to what the guys above me said..you should be pretty complete and find something that'* causing the patient to give you this rash.

OK, I had an almost identical problem with my 1994. It would intermittently stumble while waiting at a light, but if I took it out of gear (put it in neutral) it idled fine. Worked on it for a long time--nothing worked. Until...

Pull codes and look for the TCC (torque converter lockout clutch) code. On mine, the solenoid that controlled the clutch was going intermittent; it would try to intermittently lock up the torque converted while at a stop. If you are in gear, the engine is being stalled by trying to turn the wheels while you have the brake on. As the RPM dropped, the PCM would add power to try to maintain the set idle speed, and the power seemed to get the clutch to release...for a few seconds and then it would do it again.

Does your problem go away if you have it in neutral?

my 92 bonneville did the exact same thing
i had to replace the mass air flow sensor

My 94 is doing the exact same thing now....shows a code for tp sensor. Ive repplaced it twice in the last 6 months. Will a vacuum leak set a tps code? Ive just been putting it in neutral at stoplights for now.