Rich, I have the same "minor" problem on my '06 LaCrosse. There is no pattern when this will take place. It could start in the garage with I first start the car, or it may do it when driving down the street/highway at any time of the day. The instruments lights may go dim, but the highway/driving lights still are bright.
I haven't taken it to the Buick dealer, for they are noted to tear everything apart, and make matters worse. I've ran a check with various testers and cannot locate any malfunctions. Maybe Buick will finally come up with a TBS.