Dell E525w scanner problem - smeared vertical band

Hi, I've been a happy owner of a Dell E525w scanner/color laster printer, but, in the last couple of weeks, have noticed that scanning documents--whether by feeder or on the scanner bed--is creating an unusual vertical band of smeared scanning. Curiously, the side on which this occurs 'flips' between use of the feeder vs use of the scanner bed. Also, the glass is pristine--so there is not a stain on this that would clearly be the cause. I upgraded my Dell E525w to the latest firmware update and yet the problem persists. Any available solutions?