I'm guessing if the gauge "jumps" around, (by that I mean quickly, not smooth as when warming up) then it's an electrical issue. The resistance from the temp. probe, through the gauge, then to ground is changing/shorting out causing the needle to jump around. If it is not working at all now, then I would check the circuit feeding the temp. probe for good voltage, then start following it toward the gauge checking again at each connection point. If you notice and jump/drop in voltage/resistance along the way look further into that connection. I know I have a bad ground under my YJ dash that when cold (winter cold) doesn't get good connection until the cab is warm, then I assume it heats up, expands a little and grounds out. Until it does, some of the gauges read all the way to "high side" and some are completly dead with no movement at all. Once warm, or if the dash is smacked just so so (Fonzie style), everything lights up and reads correctly on the dash. This happens with ALL the gauges. If you are having the symptoms with only one, it could be the ground connection between that gauge, and the rest of the gauges grounding point. Just some thoughts. Later