My car did that two when i first put my motor in, but it says it was hot as shit and I could put my hand on the valve cover and it was just starting to get warm, so I had to burp the system as they call it, your coolant system can have an air pocket in it and makes either the gauge read weird or the coolant not flow threw the engine... So make sure its not the thermostate, just get a new one their not that expensive, put that in their and unscrew the little peck cock thing on the bottom of all radiators and just flush the system with water for a while and that should get all the air out if thats the problem, but first make sure your fans kick on and I would just buy a new thermostate if you havn't already.....