throwing belt

I have been throwing a 12 rib belt once a year for about 4 years now, its jumping forward on the car 2 ribs. I am wondering if its jumping forward on the blower while the belt is returning to the blower loose or if its jumping forward on the crank first while under tension? I cant tell because when I notice it, it has gone forward on the crank and blower, thus blowing my v-belt off of the water pump and alternator leaving me stranded on the side of the road. I am running a fixed tensioner, I am wondering if while under boost the blower is stretching the bottom of the belt under tension and the top side maybe is becoming loose as it returns to the blower. I am debating fabricating a new tensioner out of a production car tensioner that is spring loaded. The procharger tensioner is garbage as the bronze bushing wears out of it and the bracket goes crooked after a year of driving. Any suggestions? The car is an 80 malibu, sbc, carbed, d1sc with a large blower pulley, 6psi, 12 rib

I have been thinking about all of this since we talked yesterday. (I am pretty sure you are the same Joel) and I think you may be on to something making your own fixed tensioner. I think you were correct that running 2 of them would help, if not solve your problem. With only running 6psi, I think we can eliminate spinning the blower too hard. It is possible that the bracket is flexing, and building a support to the shock tower would help. But, with the info you have provided I would bet it has to do with belt tension. The serpentine belts need to be as tight as you can get them. I mean you should just barely be able to get it on. We would love to see what you come up with and how it works out for you.

Have you ever laid a strait edge across the pulleys to see if there in alignment. I have seen to many of these bolt on kits now that have the pulleys out of alignment causing the belts to jump

The blower was back in comparison to the crank about an 1/8" and I added one washer to each of the three bolts that attach the blower to the head of the motor. I'll see if it helps, it takes a summer for it to throw so I have to wait.

Joel- If it takes the whole summer for it to throw a belt, and you put a new one on every year, is it possible the belt is gradually stretching or are you adjusting the tension as it does?