Undercooling problem

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

When up to operating temperature feel both the top and bottom radiator hose. Are the hoses close to the same temperature. Does the fan turn an off properly if it runs all the time it will drop the temperature.
The sending unit could be bad for the gage. Use some brake clean and clean the terminals for the computer temperature sensor and the one for the sending unit. Both should be around the thermostat housing.

YEs both hoses are around the same temp and the fans only kicking on when the temp gauge goes up im gunna try the sensor and send,is there any way i can check these to see if there bad

I use a scanner and monitor the temperature the computer is getting.
If its its off compared to the gage I would use a thermometer you put in the radiator. Anti freeze is poison don't use your wife's candy thermometer or a meat thermometer.
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