1995 Chevy S-10 Engine wants to stall when warm

My truck starts fine when it is cold and continues to run well until I shut it off. If I return 20 minutes to an hour later, the engine starts up okay but then sputters. I have to rev the engine to keep the engine running and when I put it in gear, it jerks and coughs with little power. It gives me the sense that either it is getting too little or too much gas. This goes on for about 30-45 seconds then suddenly the engine takes off and I have no more problems with it until I turn it off and it sits for 20+ minutes. I have had it to my mechanic and he is not sure what is causing it. He has check the fuel pressure and that seems to be okay. The fuel filter also checks out as well as the fuel pump. Someone suggestion that may be it had to do with the timing device under the distributor cap however I would think if that was the problem it would do it all the time.

Any Suggestions?

Lou