Sure sounds like the starter is getting heat soaked, been there - done that. Haven't had to deal with it in years but back then there were hi-torque starters that could tolerate the heat better, pricey but they worked.
Something you can also try if you're up to a little rigging. See if you can rig up a heat shield between the exhaust and the starter. You want as much airgap between the starter and the shield as you can safely arrange.
Just did a quick Google search and turned this up:
Mr. Gasket Starter Motor Heat Shield 3678: Choose the best Starter -Performance at Advance Auto Parts
If that works it would be alot easier than fabricating your own and at under $30 would be too bad if it didn't work for you.