Source of the replacement starter motors?
New or reconditioned?
We had a batch of replacment starters for the Prescott land Rover all from the same source. All did exactly as yours after a few weeks (OK not pre-engaged versions). Turned out they were cheap Britpart imports from China - couldn't cope. If you are going to change the starter again get one of the geared Hi-Torque variety. Not cheap, but will probably last the life of the vehicle...
And have a battery shop do a load test on your battery - before replacing the starter again...
Good luck!
