doesn't sound like a bulb, rather a ballast problem. but as suggested, first change
the bulb from left to right (and vice versa) . if the same problem manifiests
even with the bulbs switched, then it is likely the ballast. if the ailment moves
over to the passenger side, then it's the bulb which is the culprit.

__________________
-- raymond~
47º 34'N 122º 18'W