The PO said it's done this for years and that if I just give it a 40 min. rest it will go again. Well, it's done this with me for 4 more years and yes, it goes fine again with a decent rest. Went fine 400 miles, rested 40 min., and then went another 100. I'll be driving 500 miles back home through more hot weather and I'm hoping I can get to the bottom of this. I have a multi-meter with me. Have a spare ECU I haven't tried yet.

Here's possible suspects that I've fixed already.

I replaced the in-tank pre-pump with the more powerful one from a later model.(could hear it humming still when problem was occurring) Had a half tank of gas.
Plugs and wires, cap and rotor are new.
AMM replaced (used)
new cat.,
new fuel filter,
hot air intake disabled,
vacuum advance disabled (got it this way)
replaced engine wire harness(used good replacement)
vacuum hoses,
intake duct,
cleaned throttle body,
new battery
replaced coil (used)

This is my first long hot trip since replacing the pre-pump, so now I'm stumped. Any ideas would be much appreciated

I've never replaced any relays, hall sensor, crank position sensor, ECU's, or main fuel pump.

Thanks,
Steve