Never heard of this but you could test by flashing the second card to 1750 just to see what you get.
I was done flash at all cards. (One 2100 mhz memory, rest 2150 mhz). When i have this "bug" i was trying to mine claymore with 30 mhs success. Then taken this card to linux rig and i have 24.5 mhs again. I think that windows 8 (i am using this) have something like "on fly" settings. I was looking for last modified files in windows and there is folder caled DriverStore and Temp. I want to install clean Windows and test again.