Serpentine belt tensioner is jumping and acting weird

probably not your exact problem, but this happened to my son. He bought a rebuilt alternator for his 318(5.2L) G. Cherokee, and he just turned in his old one for core. It bolted right on no problem, but it was shredding the serpentine belt edge. After dinking with it quite a bit, he realized the pulley on the rebuilt alternator, had one more (or one less) groove. Of course, when he went back to O'Reilly's his alternator core was gone, so then he had to go to a wrecking yard to find the correct pulley to install on the new alternator. Moral: count the grooves on all the pulleys to make sure they match.