It sounds to me like you have a deteriorating ground connection either at the sensor, or the headlights. When you turn the lights on, the grounding available to the sensor is "borrowed" by the lights, affecting the gauge.
Use a jumper wire with aligator clips at both ends, and attach one end to a good ground (pref. the batter post), and the other to the brass base of the temp sensor for the gauge. Start the engine, and turn on your lights and see if it still happens. If it does, then try cleaning the grounds from the headlights. if the problem goes away, or diminishes, you need to R&R the sensor, and clean the threads.
FWIW, a tip I learned in CC that has always been accurate is if you ever have one headlight get really dull, and the other seems a little bright, clean the ground connection for the BRIGHTER of the two headlights. It's got a weak connection, and ends up "borrowing" the ground from the good light, making it dull.
Take care,
~Chris