Pattern on Bias Frames in binning 2x2

A pattern of probable electronic nature is found sporadically in Bias frames taken with binning 2x2.
The pattern is different from frame to frame and it may be quite visible or almost absent.
In the worst case the pattern can reach a level of ±30 ADU peak-to-peak.  

The mean bias level and standard deviation apparently are not affected and stable.

The following figure provide a graphic idea of the kind of effect encountered.



 
The following bias frames were taken on the 19th of February 2007. They present a clearly recognizable pattern
and are posted here as example. We remind the reader that the effect show-up sporadically and when present it
is different from frame to frame both in intensity and shape and it can also be almost negligible.

EFOSC_Dark050_0001.fits.gz
EFOSC_Dark050_0003.fits.gz
EFOSC_Dark050_0022.fits.gz