Quite often on these cars, a failing alternator pulley causes the tensioner to bounce excessively. The pulley design is directional - meaning it's supposed to slip in one direction. If that function has ceased to work, then you're looking at other problems that will come your way.
Remove the serpentine belt and check the alternator pulley... it's supposed to turn the alternator shaft when turned clockwise, and let the alternator "run" when you counter that rotation... (use the inertia of the alternator rotor...)
Hope this helps,