It happens even with my newer GXP. Who knows. Your going along and everything dims for a sec. Weird anomaly I have heard quite a few Bonne owners notice themselves.

I have same problem too, drops to the point where my signals hyper blinks for a bit
I took it to GM on video and they said its "normal" to drove voltage every now and then

Bermuda Triangle. Ever since global warming kicked it, it tends to move with El Niņo.

In all seriousness, though, mine will do that, too. Never with a bump or a certain place, though. Testing the charging system wouldn't hurt, but I don't see a problem or a solution. Still, make sure everything is tightened down and shiny.