I've noticed, that while watching youtube videos, sometimes (actually rather
often) everything freezes. Looks like Xorg crash. It tries to recover but
screen freezes, though mouse is working and sound (probably everything else is
alive).

Dmesg is flooded with messages:
[drm:radeon_cs_ib_chunk] *ERROR* Failed to schedule IB !
radeon 0000:01:00.0: couldn't schedule ib