My camera also developed this problem. I cannot be compensated either by bias/dark correction nor by flat fielding. This is probably some problem with voltages - I could see some dependencies on power consumption, so that stronger lines appeared when camera was strongly cooled, but it was not a rule. So far I did not find any solution for this and I suggest, if you have guarantee, send it to ATIK. Unfortunately my guarantee run out thus I live with what I have.