1989 Mustang gt starter problems

ive replaced the starter twice in the last week on this car. everytime i replace it, it works fine for about 20-30 seconds, then it starts grinding, and then it breaks the lower mounting tab off of the starter housing. then it pushes itself out and makes no contact with the flywheel. its very annoying and its starting to make me mad because i know what that can do to my flywheel. is it cheap parts? do i need a spacer? (its a tko 5 speed transmission if that matters) and i have just been getting a stock replacement