Chevy Pickup that keeps dying

My grandfather bought a 1981, crew cab, V-8 Chevrolet pickup that was modified to use propane as fuel, and he towed his 5th-wheel trailer behind. He passed YEARS ago, but we were given the truck in his will. It’s GREAT. The thing has power for days, and it’s built like a tank. Even though it only gets like 5 gallons to the mile (yes, I intentionally said it backward and it might actually be 12 mpg on a good day), I would never want to get rid of it, partially because of sentimentality, and partially because it’s a great work truck around the farm.

The problem: now that it’s summer, it comes up with the weirdest issue. In the heat, it has trouble starting. Mornings are fine. It fires up within a second of trying if it’s before, say 10 a.m. However, once the afternoon hits, good luck getting it to start. All the other electrical systems work, but not the ignition. The truck won’t even turn over.

Once, my mom opened the front door and left it like that and it obviously died because the light drained the battery. Thinking this might be the issue, we jumped it, but with the cables connected to both vehicles, the same issue occurred.

Any ideas?

Offhand, this might be a case of starter motor heat soak; meaning it’s time for a new starter motor.

Next time it acts up whack the starter a few times with a hammer and see what happens. If it fires up then a starter is needed.