I am assuming tranny drive belt here. You just said top pulley, is that the engine pulley or the tranny pulley. Engine pulley should have no wobble. If it is the tranny pulley, there could be some wear where it connects to the input shaft of the tranny. excessive movement could be part of the problem. You didn't say which pulley the belt is coming off of. If it is getting flung off the tranny pulley, make sure that the support bracket on the front corner of the tranny hasn't come loose. if that bolt is missing, will allow tranny to tilt throwing of the belt.