Stalls at idle and when no gas on pedal

1994 GMC Safari, 165,000 (V4)- i've been having this problem for 4 yrs now. Turns over but stalls if I don't give gas at idle, when slowing to stop/turn, etc. The first time, had all filters replaced, MAP sensor, starter, fuel injector cleaned. Finally mechanic says it was just exhaust system clogged. Happens every 6-8 mos. Next time, new mechanic (old mechanic moved), they say it needed idle adjustment. Each time they say something different, but symptoms are exactly the same. It's running fine, then one day, stalls at idle, engine loud, car shaking/rattling, smell of fuel, exhaust smoke. Once, took it to the shop, they said they couldn't work on it, was trying to back out (giving gas while holding brakes and trying to back into street:-) saw a big cloud of smoke billow out back and immediately idles fine, runs perfectly. Next time it happens (approx. 6 mos later) on the way to shop on hwy. Same thing, all the sudden feel a release and it's running fine again. When I get there, they say sorry, can't diagnose until it happens again. Now it's same, don't you think like first guy said, exhaust clogged? If so, how can I clean myself instead of paying $300 each time for supposedly a different problem? (Btw, they never order parts and they always fix it quick if that helps?) Thanx so much for your help!