86gt5.0HO

My fuel, temp, and oil pressure gauges have been acting up for a while now. Sometimes when I drive the car the gauges read a couple of letters higher than usual in the "NORMAL" scale. Usually the temp. gauge should read "M" but I have seen it at the "L". When this happems the fuel and oil press. gauges read higher also. I'm suspecting the insrument voltage reg. Is it the little "box" on the back of the fuel gauge? if so how do I remove it from the wiring circiut?
1986 Mustang GT Cobra (Canadain)

Thanks

Rick Nagel