Is it on a windows box, with change journal enablet ?
I have seen this a few times around here, it would run a full backup, and then day day after run another full, and then run the incrementals. I waved my magic wand, which disabled change journal and Puff, the problem was gone.
Which leads me to belive there is either an error in change journal or in the way networker handles the same client in different groups using change journal.