Sorry to revive this old thread, but this is a VERY common problem in these rigs. The root cause is the small wire running to the solenoid from the ignition switch. As the engine heats up, the resistance of this wire (which runs RIGHT next to the hot exahaust pipe) increases to the point where it cannot activate the solenoid, especially if the solenoid requires just a bit more current to activate than it did when new. The heat soak right after shutting off the engine is JUST enough to make it not work. Once the engine starts cooling down for a bit (> 15 minutees or so), it starts working again. Granted, solenoids do go bad once in awhile. However, on my '90 silverado, I installed a ford solenoid on the inside of the wheel well and ran the ignition wire to this location which is much further away from the hot exhaust, then ran a heavier gauge wire to the starter solenoid. Never changed the starter, never changed the solenoid. That was over 7 years ago, and no hot start problems since.
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here, or google for "chevy hot start problem" or similar.