I had a strange problem with the temp gauge years ago. For a while, every time I put the ignition on the gauge went immediately to the middle position (which coincided exactly with the normal running position) and stayed there till I switched off. Since the engine couldn’t be hot when it was cold (if you see what I mean) I ignored it for a few weeks. One day when it realised I wasn’t going to do anything it reverted to normal function and has been absolutely fine ever since.