Hey, it could be a number of things, your rear axel(happened to me was nasty), your freehub in the hub may have exploded, or it could be your bb. i would take your rear wheel out and give the casset a spin with you hand see how it feels. if its ok it is probly your bb( recently took my square taper bb out to put an isis one in and it was in 9 bits lol). i would advise getting a more reliable hub like a Hope pro 2, of even a fixxed hub and front freewheel. Moza