Hi, have you been able to get hold of the service manual for this deck, if not go to
www.hifiengine.com and download it this will help you understand the mechanical layout of this deck. The FF/REW action on this deck is controlled by a clutch type of idler wheel, this is a complex wheel which can develop age-related cracks in the plastic components, which then destroy the decks ability to FF/REW correctly.
Before you do anything else make certain the whole of the tape path especially the pinch roller/s are thoroughly clean. Then check the capstan belt, does the tape counter work correctly when in PB mode, check belt.
Then search herein member markthefixer, he will be able to help with fixing the clutch if indeed it needs to be so fixed, this is NOT a DIY fix so you have been warn, talk to the man, and all will be well.