One thing that i could think of is when you change your thermostat your also refilling the cooling system, the cooler temps (or so the gauge thinks) can come from an air pocket near the thermostat and overheating can be caused by that also. check for exhaust gasses and if none are registered, if nothing is found there could be a very slight leak from a hose that you haven't noticed.
im no mechanic so take my advice with a grain of salt, and please correct me if my understanding was wrong.
