Sounds like good ole' GM hot start.
What I do to cure hot start:
- Make sure all connections having to do with the starting circuit are clean, and corrosion free, like kc79ta said.
-Install either a heat shield, or a starter blanket. The blanket fixed my hot start problems.
-If that doesn't fix it, you could install a remote mount solenoid, like on the old fords. Notice they almost never suffer from hot start?? The GM solenoid gets cooked every time the car runs, and if you have headers, it's even worse.
Heat causes resistance... so naturally, the hotter the solenoid, the more resistance..