Battery Ground melting....

[Art]

Hello,
I have an '84 GMC 1/2 ton, that I picked up a few years back. It runs like a top, but recently it's been hard starting after the motor warms up. When the engine is warm the starter will churn slowly or click, and the wire from the negative battery terminal to the core support heats up and melts. Cold starts are fine.
I know I must have a short somewhere, but I can't find it. I've gone all through the harness/starter looking for a problem, and have also tried a different battery. I'm beginning to think it might be something else, since the heat plays such a role. Any input would be greatly appreciated.

Thanks,
Art (Seminole, FL)