sorry I can't help you much, but I have the same problem. I have a '78 450SL, and it's lacking a little in power. oh yeah... if you haven't done so already, I would reccomend getting euro lenses w/ halogen bulbs to replace the sealed beams. It will make a world of difference at night; I did it, here's my thread (w/pics)
sealed beams to halogen bulbs conversion??