Taking my car in tomorrow for the fluctuating temperature gauge again.
My car will warm up to one notch below the middle mark in about 5 minutes. After that, it will only reach the middle mark if I come to a stop. After it reaches slightly above the middle mark (usually at an intersection), it stays there unless I drive above 50mph for a few minutes or coast down a hill in 4th or 5th gear. Then the gauge will drop as much as 1 full notch depending on the length and grade of the hill. The gauge will then rise back above the middle notch next time I come to a stop.
If I am running the A/C, this is far less likely to happen for whatever reason, and the gauge will usually stay on the middle mark. I mostly notice the fluctuation when the A/C is off.