[Tommy]

Assuming Rick is correct that this is an electric gauge problem, let me add that this is not normal. However, without a look at your wiring schematic, I can't suggest exactly what to do to fix it. ... On many temp gauges, the gauge is actually showing the electricity flow through the temperature sensitive resistor that is the sending unit. If changing the current output of the alternator or the current draw of the headlights affects the current flow through the temp gauge, I'd suspect a less than adequate ground somewhere else is causing electricity to take that route. .. One quick test would be to find the ground for the headlights and run a temporary ground from there directly to the negative pole of the battery. Then turn on the lights and see if the temp gauge changes. If it doesn't, then you have found the problem.