Most likely a miscalibration problem due to dust/dirt/smudges on the top 1-2 inches of the scanner bed's glass (that area must be absolutely spotless) or improper orientation/placement of the film holder. If it is not that, it could be something more tricky to deal with like dust on the mirror system (try putting the scanner on its side and running a few scans to vibrate the dust and see if it moves or falls off the mirror). Worst case would be a bad section that develops on the CCD.
Doug