2000 GMC Sierra

Hard starting is not an easy problem to solve - especially when it goes away and is temperature related.
First - check and monitor the fuel pressure. The pressure must be checked at several different times during a "run cycle". Hook up fuel pressure gauge. Turn the key on - check. Then start engine (or try to start) - check. Let engine warm up - check. Then drive around for 15 miles or so - check. The fuel pressure should never go below 50 PSI. If it does you could have a blocked fuel filter or a leaking fuel pressure regulator or a failing fuel pump. Also, the ground for the fuel pump may be corroded. It connects to the frame cross-member just before the gas tank. It is the black wire that breaks out of the wireing harness in that area - and - connects directly to the frame. Try cleaning it up and re-terminating this ground, they tend to corrode.
Second - check the quality of the spark with a spark tester. They cheap and are available at any auto supply store or online.
Third - there are two sensors that relate to temperature - they are the IATS (intake air temperature sensor) and the CTS (coolant temperature sensor). These sensors change resistance according to temperature. If they fail they can cause this problem. Testing these sensors requires an ohm meter and an accurate thermometer. (Sometimes it is better to just change these sensors and see what happens).
Fourth - it could be an oil pressure problem. Test the oil pressure - do not rely on the sending units' mesurement. If the sending unit is failing it could cause this problem.

BTW - all of this is assuming that it is a gas engine.