You probably lost a ground somewhere in the electrical circuit. My 66 coronet would lose the temp and fuel gauge when fuel ground would lose its ground.

56 Plaza 63 D100 step side 67 Coronet, 68 Roadrunner, 69 Super Bees, 69 Coronet 500 convertible, 70 Roadrunner Post, 79 D150 360, and a severe case of Mopar a,d,d