It's the power outlet probably, the display cable (or an internal component?) is not properly shielded and you get interference. Try to relocate - and you can change the fixed refresh rate too.
It can also happen if you have too many appliances plugged to the same socket. It could be worth trying to plug the monitor somewhere else with an extension cord.
Thank you! Both will be tried soon - now I don't want to disturb the "roommate" chick. 
(Relocation possibilites are quite limited in this room. Maybe I should move to another company. 
)
Tried both - still flickering. What's more, VGA ---> DisplayPort + applying EMI/RFI filtered connector were also ineffective (at least the annoying "ghost image" disappeared with the first, go for it Rotoi/Chulk/Friker
). Since I used my own computer several times in the office, it also started to flicker - or at least I think so...
Solutions:
1, My eyes and brain are playing a stupid game. Nobody else detected the problem while watching my monitor. Even I start to feel it only after 10-20 minutes.
2, There's a strong magnetic disturbance. Although it mustn't have an effect on TFT screen...
3, 60 Hz is not enough for my eyes. Another useless factor for TFT, but who knows?
4, Unknown effect, causing a general backlighting error of HP screens (?). Sounds silly.
Poll