94 Chevy Silverado stalling

I posted in a older thread but I will start a new one. I have a 94 Chevy Silverado 4x4 with a 350. Since I bought it 9 months ago it had a problem with stalling. It was especially bad when the AC was on. On the highway it works great but stop and go traffic it will stall constantly. I had the local mechanic look at it. He tried the IAC valve, ECU, Power Steering pump, and miscallaneous ground. I replaced the plugs, wires, and cap. I took it to the local dealer and after 8 hrs and about $400.00 he said we can't figure it out. He did suggest the Distributor. Anyone care to offer a suggestion. I'm thinking about wedging something in the throttlle to idle it up a few 100 RPM's. It seems to idle very slow but as long as it is not in gear it will stay running. HELP

Try cleaning the throtle body.

The throttle body was replaced with a know good one and the problem was the same.

Here are some other things to try. Oxygen sensor, Test TPS at idle, engine coolant sensor, Fuel pressure, check fuel trims, fuel injector balance, EGR, vacuum leaks, crankcase vent valve, low compression and cam lobes.