This brings up a point........HOW ARE they failing, Denny? Is the coil going open or the contacts failing. OR IS THE ENGINE just rattling them apart from vibration?
The last thing that comes to mind..........same starter? Maybe you have a "partially shorted" starter solenoid which is drawing way too much current. If this is a mini starter it would be easy to check current draw, but I don't know what the "average" is. You'd have to check several others to get a baseline
If you are using the old original type starter, this gets more difficult. If you are familiar, the old starter solenoids had TWO windings.........a very heavy "pull in winding" and a "hold" winding.
The "pull in" winding is in series with the starter internally, and it only engages during pull in. Once the solenoid contacts are closed, it is bypassed, leaving only the "hold" winding.