You have the multiplex gauge system (or whatever it's called), so every gauge is plugged in in series after the speedo. It could be as simple as one cable from one gauge grounding out or loosing contact. Try disconnecting the lead that comes out of the speedo and see if it still does it. Then plug one in at a time until you see what's happening. Or you could bring it to a dealer, that is the CURRENT system so they do have ways to test it. The 03 and earlier is the old style that the dealers have no idea how to work on.