Driving myself nuts

I replaced the head gasket a few weeks ago. I did everything by the book aka haynes new gasket new bolts right torque and sequence. Since I got the car back together I have not been able to get the right temp according to the gauge im running about 40 degrees under temp. Every once in a while the gauge will slowly rise up to 30 degrees over temp. Ive put a new thrermostat in the car acctully twice one auto store then a dealer. Now the car runs good like it did before ive put 900 miles on the new gasket and my eyes are glued to the oil, coolant, plugs, and exhaust so im sure the headgasket is right and I dont believe something wrong with head/gasket would cause overcooling would it. Now 2 days ago my car sat for about 5 hours then we went out and got 1/2block away from my house and the car said it was at the temp its been running at. Now this would lead me to think its the sensor/sensor plug/gauge, because even with no coolant its not possible to heat a car up that fast. Before I spend anymore time i was wondering has anyone had this problem If i replace anymore parts and it does not fix it im gunna go nuts PLEASE HELP

you did burp the cooling system right.