These cars are sensitive to bulb wattage. Variance in non-OEM bulb wattage can cause problems. If it really bothers you, pull the cluster and remove the bulb. Otherwise look for Osram or Narva (or other OE sounding brand) bulbs of the specified wattage. Might find some in the pick-n-pull.

It's curious that it only happens when the headlights are on so maybe it's a bad ground at the tail lights.

Sixto
87 300D