> If you can determine the spacing of the lines (e.g., every 7th line is dark),
> that will help a lot.  The period of the noise will help define its source.
 
There does not appear to be a discernable pattern to the noise.  There is a
band of distortion about 50 to 100 pixels high on the scanned image.  Actually
Bio-Rad suggested getting a filtered power source.  I did not want to shell out
$500-$600 and discover that this problem persists.  I the banding does not
appear to be from electrical power noise.  Presumably the building's power
source is not as much in use in the evening hours, however, the problem
persists at all hours.
 
Paul Anderson
Thomas Jefferson University