Before I tear into my 450 SEL (1978) dash and replace the temperature gauge is there a way to check its calibration. It all started one day as I started the car and it read 250 F. The engine was ice cold. I kept driving it until the weekend to work on it. It became better. It starts at below 100 F and reads between 175 and 250 as it warms up. I changed the sending unit, the one towards the back of the engine. No change. I changed the thermostat just to make sure it is operating at 167 F. No change. Should I change the temperature gauge? Is something else here that I am missing? Thanks for your help.