I replaced my 1984 speedo with a 1987 145 mph speedo, it read 25 mph too much so I had an adapter made. Everything worked fine but now the needle goes up but comes down and sticks at 5mph. I took the plastic off and carefully manipulated the needle to below 5mph, and everytime I do manipulate it, it goes back to where it should be. It's only when I drive it that it sticks at 5 mph. Am I missing something or should I just replace the speedometer cable?