Here is a strange problem: I swapped a Milodon high flow 180 degree thermostat into my 70. The temp gauge rises up quickly on warm to about the 3/4 mark, then falls back down to the 1/4 mark. I have never had any car do this...... All suggestions appreciated!
Sounds like it is sticking closed. Halfway on the stock gauge is around 200-210. About 2 lines up from cold is 180. A 180 stat will begin to open at 180, and be fully open at 200*. I would try another stat, and put a real temperature gauge on it like an Autometer. Just my .02
