Mark,

I would not rule out a bad ground. I had a GEO with a bad ground a few
winters ago, that exhibited the EXACT behavior that your bus did. I did
not enjoy driving with the high-beams on. :-(

Clean your ground contacts and you should be fine. High-beams are often
on a different circuit from low-beams so you can at least drive home if
all else goes kaput. (check the Bentley for your model year)

Best regards,
Sam