1990 Silverado Dies when warm

350 TBI starts and runs great for about 20 minutes.  At 20-30 minutes just shuts off.  After that, no spark but good fuel at the injectors.  If the truck sits for a few hours it will start and run for 20-30 minutes and die again.  New cap, wires, plugs and coil.  Checked pick-up coil, reads 700-800 ohms.  Ignition module passes test when hot at AutoZone.  Did the full troubleshooting routine out of Haynes manual, all good. 

No stored codes when this happens. 

So now that everything that it could be is eliminated, what else would cause the engine to die after 20-30 minutes every time?

Thanks for help and advice.

Monte