1998 Mustang speedometer problem

hi i am having some issues with my 98s speedometer. I had bought the car from a person who had got it from a salvage yard. minor front end damage. car runs like a top and the motor has been replaced. My issue is alot of the time when i start to drive my speedometer wont register right seems slow but if i give it a tap it jumps to the right speed. today it was stuck at zero and the tapping didnt work. I finally got it to work while tapping the cluster but it takes forever to get to the actaul speed while traveling 55 it takes forever to get there and when i slow down it is just as slow. The guy said he took it out and turned the odometer back like an idiot to match the engines mileage. please help