Sounds like a pinched wire , either in the hand piece or mount , do you know how to use a multi meter ? if so just start at the mount and test every wire from start to the end , inside the hand controller , this may take a while and the covers need to come off the moters and hand controller , but you may be able to find the pinched wire ? ,
I have seen this beforte in a Celestron GOTO mount ( it went hay wire before it stopped as well , the pinched wire was replaced along with a fuse and all was good ) , it was a wire in the mount , the RA drive if my memory serves .
Good luck /.
Brian.