Check the grounds in the left front fender and clean them. The headlights are strong enough to rob the other lights of ground.

Once, our '90 744 TI would stall when the headlights were turned to high beam - its in the FAQ!

Also check other grounds in the left front, my car had a ground by or on the cage that holds the anti-lock brake components.

When checking grounds, items mounted in plastic, such as lights have ground wires, and therefore grounding points. Items that only have a power wire going to them obtain their ground through contact with the engine or body.

On a car of your's age, you should consider unplugging wires, cleaning the contacts, adding dielectric grease and zip tying any that you can.

Bad connections can drive you crazy!

Do one connector each week - that way if you have a problem, you know where to look.