You are running an electric temp gauge. If I recall a dead short will peg the gauge on the hot side, and an open circuit will leave the gauge showing cold. Since your reading cool, it could be that you have either a bad wire running from the gage to the sender, or a bad power supply from the ign to the gauge. Your gauge also has a reference ground that can be contributing to problems. My guess is that both stats are OK, your engine is over-cooling maybe just a tad, and that the dramatic changes you are seeing are the result of an intermittent electrical problem in the gauge's wiring. I think I'd put a meter on the ign post of the gauge, with it wired and running, and check voltage to chassis ground.
Doc Vette -- We miss you?