Ultra 150 starter motor problem
 

Hi guys, I have a problem with starter motor polling out, this is the 3rd time and it’s p..ing me off, normal starting ok, than if the battery is a bit low it polls out, I remove the starter motor check the damage, clean and replace 4hrs later, works fine for some time and goes again, have tried a new starter motor and rebuild 2 others,, why does this keep happening? Any ideas