Did you test the old starter or did you just have one put in and it worked?

I am wondering if a) it is not really the starter, but the wiring or b) it is the starter, but the poor wiring is killing the starter. Corroded wires can cause some serious choke on amperage and low amps going to high amp motors is not a good thing.

Did you check the battery and cables last time?

In the end, the starting system is pretty simple. Battery, starter and the cables that go between.

Good Luck,
Michael