I think you have a voltage drop issue and it is causing your early starter failures. What happens is the positive cable (negative as well) corrode inside the plastic coating on the cable. This causes high resistance and voltage drop at the starter. You may have 12.7 volts at the battery but only 11 volts or less at the starter. It may still work but it will shorten starter/solenoid life big time. I would replace both cables with new OEM cables or have high quality ones made. The Jippy Joe ones at the parts stores won't cut it.