I get the same problems as described before with two different monitors. Using
an up-to-date Arch Linux with kernel 3.14 (also tried kernel 3.15rc2) with a
hd3470 card. Problem occurs only, if an external monitor is connected at
resolution 1920x1080@60Hz in single mode (extended mode makes no problems).
Funny enough when using 1920x1080@50Hz in single mode everything seems also
fine. By switching back to Catalyst the problem doesn't occur.

Found some similar threads:
https://bugs.launchpad.net/ubuntu/+source/xorg/+bug/1209315
https://bugs.freedesktop.org/show_bug.cgi?id=42621

By disabling color tiling and dpm the phenomenon appears only when switching
between modes (monitors on / off). So in this case when starting the laptop
with closed lid everything is ok, until switching between modes.