Weird over heating problem

[desertdogg]

So I have a 1972 chevy c10 that im working on and Ive had an over heating issue so Ive flushed the radiator and currently got some prestone super clean in it to try to get all the sludge build up in it. Anyways I have noticed when my headlights are on it wont over heat I can literally run it without the head lights on and I will watch the temp gauge when it looks like the gauge is at the maximum heat range I will turn on the lights then the gauge temp goes back to normal running temp. What the hell ?...anyone have any ideas what might be going on here ?...thanks for any input