Heinz,
it does seem that the problem is, as you suspect, uneven friction due to the disc and pad not working well together.
Some web research led me to
THIS article: it seems logical and covers a number of other brake related issues.
So, I think the answer is simply to have the discs media blasted, not skimmed, and then to bed in new pads on to the clean discs.
As an aside, after washing the car I try, whenever possible, to dry the pads/discs by going for a drive and doing 4 or 5 firm stops from 60-80kph, just to the point the ABS cuts in, about 0.8g. The Plus 8 discs rust very quickly, like the discs on the AMG and my bikes. From memory the discs on the roadster didn't seem to rust as easily/quickly.
Good luck with this.
