Truck won't start when hot

[Bruin_26]

Hey guys, so I'm having a frustrating problem with my '72 C20. It simply won't start when it is hot. If I turn the key after having driven around for awhile it won't do anything! No turning over, no clicking, nothing. Interestingly, if I turn the ignition to the "on" position then bridge the contacts on the starter with a screw driver it starts right up without a problem. It has a 350, 4 Barrel quadrajet, HEI distributor, rebuilt starter, good battery, new ignition switch, new battery cables.
Looking forward to any suggestions and advice and thanks in advance!