Have you tried to cool the starter down with water when it doesn't start? Just to see if it cools faster and will start up again. This will prove that it is a heat problem.
the headers on the 289 in my '66 Mustang used to cause the starters to get cooked in no time (and then I'd get the same 'no cranking when hot' problem).
I simply painted the next starter white before installing it and I never had any more problems (believe it or not).
