LaserSoft Imaging

Hi there,
I have recently bought a Reflecta Digitdia 5000 bundled with Silverfast.
I get good results but from time to time I get scans with vertical bands on them which are very visible on slides with blue skies. See attached image. On one day it happens on many slides (but not always), one day later when I scan them again they look OK. So it seems to happen a bit random.
Does anybody know what causes this and what I can do to prevent it?

Dear Customer,

for me this banding looks like a hardware issue.

Are the bands along the direction of scan or orthogonal ?

If along the scan do they repeat at the same pixel location when it happens ?