72 GMC Sierra Grande starting probs..help?

[philintexas]

Posting for my father...he has a 1972 Sierra Grande pickup with a 400 big block bored to 402. Beautiful engine!

At any rate, the truck has a starting problem that dad cannot figure out. After driving the truck for a while (15 mins or more), when you stop, the truck will not start again. What it does:

<> Put key in ingition and turn and nothing. Starter does not engage.

<> Power IS working. When key turned to first position, everything goes "hot"...fans turn on, etc.

<> Switch is brand new. Wiring is all new. We thought it was a ground and replaced everything.

It just appears that when the engine get's hot, something disengages and the starter is not powered. When it cools down everything takes off like normal.

Dad's at wits end on what's causing this. Could it be the starter? He had it checked out at the local mechanic and it was recently rebuilt and the mechanic said it's good to go. Any other little thing in here that could be causing it?

Thanks so much for any help!!