300SEL Electric Gremlin

OK, I have a 1988 300SEL with a strange electrical problem. Whenever I turn on the headlights my speedometer dies. All the other electrical systems work just fine, but the speedometer drops to zero. After about two minutes, it jumps back to normal, however it keeps droping down then jumps back up. If I turn the lights off, the speedometer works great. I am thinking is a relay somewhere, but I do not have the electrical grid and I am not sure where to start. All help will be greatly appreciated.