(Additional info to comment #12)
> Interesting: If I connect only one monitor (either on cable 1 or 2), it works well.
It is not working well, but in only blinks in longer intervals. After resuming
from hibernation, it started blinking more.
(In reply to comment #13)
> 2. Disable acceleration (Option "NoAccel" "True" in the device section of your xorg.conf)

After starting Xorg with NoAccel option, it is not blinking. But:
1. why don't use acceleration on accelerated graphic card?
2. I can't use rotated monitor, because it requires acceleration enabled.