The problem is most likey the output flange nut on the transmission. If it is not torqued properly, or loose, the speedo will act as you have described. The socket you need is large and notched, and I can't remember the size. As a last resort, or fisrt resort, you can use a punch and hammer to tighten the nut. Probably the correct torque is not as important as just being tight enough. This is a common problem that is usually mis-diagnosed. Good Luck.