Gets hot then starter is very slow turning over.

I have a 74 Duster 360with Patriot headers and the large original starter. Once it gets up to temp the starter turns very slow. Usually wont start the car till it cools down.
From what I read it's the solenoid, but is it the one on the inner fender or the one on the starter itself?
Does a mini starter help? If so what's the best value mini starter?
Anything else I should look for?

Cley