Well a few ideas come to mind. If it's your starter, that can be tested with a "starter draw" test---however, that can give you a false clue because if it's your engine that's binding internally, that would put the starter under great strain.
Was there ever a heat shield next to the starter that maybe someone took off?
Does the engine run very hot? That could cause internal binding, too.
Who did this rebuild? If you got the rebuilt starter from a chain store, it would be suspect.