Quite often the problem is actually a worn cable housing with rough spots where the cable/housing assembly negotiates bends. If that's the case, replacement of the whole works is the only fix. The cable hangs up on the rough housing, winds itself up momentarily, and then snaps free- - - -causing the erratic needle bouncing. The Freightliners I used to maintain for a small trucking company had the same problem with tachometer cables.
Jerry
