I decided to give the engine a quick wash over. After that I have noticed that the temp gauge started to fluctuate. I believe the wash started it, but I am not 100 % sure.
I have since driven about 700 miles, and the temp gauge can stay normal for hours before falling down over half way to the bottom. It then comes back and it can fluctuate like that for 10 to 20 minutes before returning to normal.
As far as I can understand, the engine runs fine while this is going on.
Can this be anything other than the termostat? Bad wire? better radiator cooling effect after cleaning? And should I get it fixed? It can stay fine for several hours; in yesterdays 14 hour drive it only happened twice.