Got this from another forum. This is a common problem.
You have "dying gauge syndrome." Here's how to fix it: Pop off the defroster vent. It has no screws or anything in it. Just clips. From there, you'll find the screws in the top of the dash that hold the instrument cluster cover in place. Keep pulling stuff around the instrument cluster until you get to the cluster itself. It pulls straight out. On the back, you'll see a male connector that looks like a computer connector. Take a pair of needlenosed pliars and twist each blade 2 or 3 degrees. What you are trying to do is cause the male portion of the connector to make firm contact with the female portions, so that when you go over bumps, the vibrations don't shake the connector loose. Gently put the gauge cluster back in. If it won't go, don't force it. Turn it over and find the blades that you have perhaps turned too far and correct them. Reassemble your dash, and voila: Problem fixed. Good luck.