1989 GMC Vandura stalling

How long since a tune-up? Any applicable trouble codes? Your obd1 so you can check some trouble codes, yourself, as long as the check engine lamp works. Does it stall at idle or going down the road? The basics for a rough idle, check for a leak of some kind, fuel--egr--vacuum leak.

Also, it only stalls after driving for a couple of minutes and sometimes when put in reverse. Mechanic only showed map sensor as being faulty on scanner but replaced it...still stalling, though.

Usually with MAP trouble codes the problem is not the sensor itself, something else is causing the sensor to sense incorrect maifold pressure which would be either high or low vacuum. In my case when I was getting a MAP trouble code and an oxygen sensor trouble code I found that one of my TBI injectors wasn't working and when I opened that up I found the whole fuel system on the throttle body was full of crud and a gel. Further investigation found I had water in my gas. Water in the gas also causes stalling and running rough.

Extension of my previous answer. In the case I steated before I'm guessing the IAC was opening the throttle a little to try to get more gas or keep the engine running but it wasn't getting more gas just lowering the vacuum hence the MAP code and the oxygen sensor code was caused by a bad fuel/air ratio so the MAP sensor code and the oxygen sensor code weren't the sensors themselves being bad, the sensors were actually doing what they were supposed to do.