3.8 won't start after stopping

I have an 85 Cutlass Ciera with a 3.8 litre in it. I replaced the starter 3 months ago and ever since then the motor won't start again after its been driven a while. First thing in the morning starts great. Drive it for a while-stop-go shopping come out and it won't start. If I put it to the floor, pump and curse it will tease me. Pretend it will catch and die after pumping and cursing for about 15-20 it will fire up and almost blow a head gasket. I have had it GM dealer diagnosed, new plugs, injectors cleaned, it comes up clean under computer diagnosis. Any clues before I take a 45 Magnum to it?
Thanks

Check the timing, make sure the choke rods on the carb aren't bent preventing it from working properly, if that's all good get a compression test and a fuel pressure test. there are alot of reasons for that to happen but it's gotta be narrowed down a bit.