Don't know what the problem is or how to fix it. It must have been the weekend for tach problems. In my wife's 525iT, the tach went , very jittery during a 1 mile drive ,at least once it circled around to 1000 or 1500. It stuck for a few seconds at 5K when I shut it off, 10 minutes later it was fine and the problem hasn't reoccurred.

Warren
91 318is
95 525iT