Coolant temperature gauge wonkiness

It is more than likely the sensor. You'll need to buy a new one as the sensor and gauge are matched.

Check the sensor, wiring connections, wiring and gauge. If you can get a second hand sensor from a car that is the same as yours it may when installed show, a lower/higher reading, if the reading is steady that it is the sensor that has malfunctioned. If it is the sensor then you need to buy one. The sensor and gauge are matched. Another sensor can work but the reading can be lower/higher. If when you put in a known good sensor and you know the wiring is good and the gauge raises and falls then it's the gauge that's malfunctioned. If you know the sensor and gauge are good then it's the wiring that is damaged. Air in the coolant and a collapsed coolant pipe, sticking thermostat may also cause the gauge to fluctuate. I do not know about your car. Mine is a 1996 A6 1984 cc 4 cly engine which has 2 sensors one for the gauge and one for the computer. Getting to to the gauge is a long job.

This happened with my RS6 a few years ago. I had it fixed at an Audi shop - wasn't expensive. I think it was in the sending unit, but not sure.

Dave A8L

Almost certainly the sensor. I've had several replaced. Here's a DIY article:

I had a 1986 4000CS quattro where the coolant temperature rose to normal, and then fell down towards C. If I slowed down, or followed someone closely, the temperature rose. It then fell again if I was in clear air. Solution was a coolant flush and refill.

Steve