I have a 96 k1500 with a vortec 5.7l. I have completely rebuilt the engine and it has a new transmission. The engine and tranny has 1300 miles on it. It has a new distributor, cam position sensor, crank position sensor, cap, rotor, plugs and wires. When the engine is cold it fires right up but once the engine warms up it is very hard to start. It almost sounds as if the battery is draining and it will die but it will eventually start. The battery and starter is new too. It runs good once it starts but it turns over really hard when you start it when it's hot.

I have no idea what's going on and can't find anyone who knows what to fix.

 

I'm thinking it has something to do with the starter taking on too much heat. Try wrapping it up with some heat shield. Or something to block the heat from your exhaust