starter problems

I have a 95 k1500 with starter problems. It started out as breaking the outside bolt all the time. We drilled all the way through the block and nutted both sides. It held for a couple years. A few months ago it broke the inside bolt. Replaced it with a new one. Now I'm having problems with them staying tight. The past few days they've been tight but somehow the starter is coming out of place just enough I have to loosen the bolts pull down on it as far as I can tighten them back up then start it. All it does when I try to start it now is grind. I'm guessing the teeth are worn and I need a new starter but I think there's another problem underlying somewhere. Thanks for any help.