92 chevy truck 1500 4x4

I have 92 chevy truck 200000 miles. and when the truck gets up temp. it will alomost stall out when u let the gas off. It only happens when ur going about 30 m/hr it will almost quite. RPM's go up and down and will level off till u let of again. If ur driving in the bush it will almost cut out when going slow like 15m/hr I have changed fuel pump, filter, distributor, spark plugs and wires, exhaust manifold on pass side as it had a crack in it, intake manifold gasket, thermostat, and antifreeze. check engine has never come and the local mechanic said that the computer wouldn't help me till it did. can some1 help me out. Thanks mike
Thanks to juggalo my truck is working 100% better, these machanics in town are a joke but thank - you juggalo.