99 honda civic temp gauge keeps going up and down slowly while driving?

I just had my engine replaced with an engine from japan because I had a blown head gasket and my car would overheat once the radiator ran dry. After having my engine replaced the temp gauge goes up and down after driving for a bit (like 20 min into the drive) and especially when I go up hill but the needle never goes all the way up and the car doesn't overheat. Any suggestions on what it could be? Cooling sensor, thermostat, thermostat sensor? Please help!
 
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