Starter heat soak ????

if it cranks normally again once it's cooled off, then I'd say your diagnosis is bang on.

X2, I had the same problem, either go with a mini starter, if that is the problem, or you can wrap the starter motor with some high temp heat wrap. I did that on mine and so far so good.

I bought a 12" by 12" piece in a package at NAPA and put it just around the motor itself.