why does the truck intermittently not start usually when engine is warm.

We have had the truck to two different repair shops over six times. One was a GMC service center. They cannot find the problem.

That's hard for me to believe!! .. While the truck will NOT start .. What's missing FUEL OR SPARK? .. Those are 2 pretty simple test for a 'mechanic' to make using the appropriate equipment.. Perhaps you just haven't found the right one yet... In their defense I will add that intermittent problem can be very difficult to diagnose... Good luck!