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That's a fairly expensive fix. IMHO it would have been cheaper to install a heat wrap around the starer and/or replacing the positive battery cable with one of a larger gauge and better insulation.

Thanks, Denny. I'm just an old shade tree mechanic and if I have any expertise at all it's in the electrical end. At one time or another I have encountered all four of the situations I posted.

This is all good stuff guys, but it doesn't help to solve the problem.
Since he has tried at least three different starters, IMHO, that eliminates the starter as the problem. IMO that leaves four...

Are you using the same solenoid that was on the original starter?

From your description, among other things, it sounds as if the bendix is weak.