Additional specific info:
I'm using extended desktop on two DVI connected monitors (the card has one
connector and you have to use Y cable to connect something). Even if I switch
to cloned desktop, it blinks too.

Interesting: If I connect only one monitor (either on cable 1 or 2), it works
well.

Both monitors have resolution 1680x1050.

My ideal monitor position would be desktop assembled from 1050x1680 (rotated
left) and 1680x1050.

With fglrx it was good working...