how does it feel when the speed dometer is jumping up and down?

How does it feel? Don't know, my speed dometer just sits there.

how do you reapair it?

Sonris, what do you mean, really? I would feel like I need to have the speed dometer checked out by a mechanic

To do it yourself? personally I couldn't repair it myself, a 1996 Mercury has the speedometer operated by electrical signals sent from the transmission, it's not a mechanical device like a cable that turns like in 70's cars

This might do it --- If you think you are having problems with your speedometer because of tire or fuel changes, the the best way to solve it is by resetting the Programmable Speedometer/Odometer Module (PSOM). You can do this by pressing the SELECT button found on the upper right corner of the speedometer, then you select zero for the trip odometer. Next, you press the RESET button (typically on the upper left side of the speedometer), and once the trip odometer is displayed, you then press the SELECT button again to see the odometer value. It is also a must to change the speedometer's internal conversion constant if you have changed tire sizes because this will enable your speedometer to calculate your speed accurately.

huh?

Huh and Huh twice to the question first and then the answer from tenspeed, I would also think if my personal speedometer jumped up and down my back would hurt. What a minute he talking about a speed dometer, isn't that a question for the bakery department? This has to be my shortest answer or inquiry.