Has it got a voltage stabiliser? Check and replace, also the earths on the instruments.
It could just be the gauge. A couple of years ago my temp gauge started reading high. I replaced the voltage stabiliser, the sender, the thermostat, and the water pump. Coolant temp measured with an infrared thermometer shows it to be completely normal but my temp gauge shows around 110C. I've decided to just live with it as putting a new temp gauge into a 40 year old dash would look silly.
