keep blowing belts, Tensioner

For the last year I've been blowing belts on my stang. The belt will immediatly jump one rib towards the engine on the AC pully, eventually burn out and snap.

All pullies are seemingly in line and bolted in tightly. Had a shop look at it and they couldn't figure it out without costly analysis, I decined.

THe mechanic and me think that the problem might lie in the tensioner.

The tensioner feels like it could be tighter. When the belts on, the tensioner can be pushed down (supercharged application) about an inch, then it is fully tight (which I know is not optimal).

Some history: when this first occured I found that the powermaster alternator located below my smog pump was loose. I tightened it down and everything seemed to be fine. Reciently, I put a new undamaged belt on and it jumped so I once again looked at the alternator. Tightened it down some and the belt tracked straight. Drove the car twice, then hit the freeway to drive 3 hours to my hometown. Car slipped the belt mid drive 30 minutes down the road. Towed to shop and they could not find any serious allignment issues, but said the alternator appeared to be a little in front of the others.

Drove home and slipped the new belt, but managed to salvage it and made it home.

Like I said before, the mechanic said it might be a tensioner or a design flaw in the alternator. I figure the tensioner because the alternators never caused problems before, but it does look slightly off.

Any ideas