1992 buick stalling problem

I have a 92 buick regal with the 3800 V-6 with 60000 miles on it. Sometimes at an idle the car will run rough then straighten out. Lately it will do this when I decelerate then stall and I have a hard time restarting it. When it starts again it feels and sounds like its running on 3 cylinders then stall again as soon as I let up off the gas. This will go on a few tries then all of a sudden the car runs fine. It can run fine for a few days even a few weeks then it acts up again.3 mechanics and Nobody has been able to figure this out. I should mention that a year ago this was happening more and more and a mechanic blew the motor trying to duplicate the problem. I have since had a different used motor put in by a different mechanic and a year went by with no problem and now its acting up again. When the new motor was put in the mechanic told me that the PCM was way off and he adjusted it. This has recently been rechecked and it's fine but the stalling problem still exists. Any ideas what the problem could be and is it a common one?