Sorry for the delay, but the at the moment I seldom have access to my home PC, have to spend a lot of time at the office.
Just to add one little detail which I did not noticed before - the lines change their position sometimes. As far as I could see, this is not/not entirely has to do with restarting the device. Sometimes after rebooting the HMD the lines are at the same position they were before, sometimes their position is changed.
At this very moment (I had only like 20 free minutes to test it again) these dark lines seem to have faded and are not so annoying as they were, but they are still there.
BTW, I saw a thread about removing some filter from the HMD to make the image brighter - does it have something in common with my problem? I hesitate to do it since I plan to sell the device, but if this will be the only way, you know...
UPD I also will try to reflash the firmware as soon as I get at least half an hour to do it. Lets see if this will mitigate my issue.