I was having problems with the water temp gauge on a '97 2500 Cummins. The reading would wander up and down the scale over a period of minutes. First, replaced the sender. Still wandered.
Some knowledgeable folks on the group here told me the wandering problem was well known - change the thermostat. Did that. My gauge is steady as a rock now...... LOL...... at 160 degrees F.
Looks like I'm gonna replace the dash gauge. :-) Any gotchas to keep a watch for?
Justin