Ok, I am stumped.
I have a 99 Ranger 2.5. Runs great, heater works, t-stat opens and closes as it should. When it gets cool out (60 degrees or under), occasionally the temp gauge will drop down to the lowest point or close on the gauge while not in motion (traffic lights). Once you get the truck moving again, it goes back up to the middle. It's intermittant. Coolant is fresh, conector at the temp sensor is good and I put dialectric grease on it. If it's hot out, it's fine.