My alternator belt on my 82 Jimmy keeps coming loose. So loose that it won't charge the batteries. I use a screwdriver to move the alt to tighten the belt, then tighten the bolts. I'll start the truck and it starts to charge the batteries. After 5 Min. I turn off the motor and the alternator has slid towards the block, loosening the belt again. All the bolts are tight and I have to loosen them to re-adjust the alternator. Anyone ever have this problem Any ideas to fix it?