Originally Posted by tabetha

You can test the gauge by earthing it out with the temp sender wire where it will go to max, as you can see the temp go up then fall back the stat is ok, so most likely the sender, these were on normal sierra mk1's adjustable, on the actual gauge, had a sliding mechanism bit on the back of the gauge, never needed it myself, but if it comes to it, but I would also try a new sender first .
tabetha