+1 for earthing problem, I recently removed my earth lead, and sanded the engine and body contacts. If you have access to something that will read your coolant sensor (not temp sensor), eg ECUTALK, or PFC i guess. you can monitor the temp and compare it to what your gauge is showing, as they are separate sensors.
Talking about temp gauge, I just bought a 2nd hand instrument cluster from japan, $25, as Nissan no longer list the temp gauge, as i discovered with much testing on and off the car, and replacing of sensors, that my temp gauge would sit in the same position (about 1/2 way) from 70 to 120 degrees, then resume. Not real handy. Guess it must have either broken some windings inside or worn a rut in the windings ?? Anyway, hope the new gauge doesnt have this issue.