I had that problem, though just with the temperature gauge and found it was a (chassis?) ground cable to the transmission/engine interface that was causing the problem. I re-worked it and the gauge works fine. Whoever who had done work on the transmission hooked the ground cable up to the wrong size bolt and it was functioning intermittently because it was loose. The car had been starting only part of the time, and investigation (sparks)revealed the problem. The gauge now reports without waving at me. In other words, the ground in question may be somewhere else besides the dashboard.