Gauges go to full when headlights on

On my 65 valiant when I turn on the headlights my temp and fuel gauge go to full. If I turn off the lights while running the gauges read correct. I've had these gauges redone twice by instrument specialties. Plus I added a new headlight switch within the last year. I also bought the updated instrument voltage regulator when I had the gauges redone the second time. So what's wrong here?