i always mark a tooth on the belt to each cam sprocket and also on the crank sprocket so there is no room for error when reinstalling the timing belt. however, if your belt is coming off the cam sprocket, next time you put the belt on, run the engine with the timing covers off so you can inspect all the pullys and sprockets for warpage or mis alignment. when you run it without the pullys you need to put crankshaft pully back on so cranckshaft sprocket does not walk off.