@Francisco,
Thanks for the clarification.
This “frosting” screne problem – does it freeze the screen apart from
scrambling, and forces you to reboot?

I found that on 3.13 I was able to use my laptop sometimes for hours, or even a
day, without this happening. But it eventually always happened.
I am on a Thinkpad W500 now with two GPUs. I switched to Intel for the moment
and everything is fine. I will try again switching back to rv635 to try open
drivers. Maybe soon, as you report no problems.

However, I came to appreciate the temperature drop, battery life increase, and
better Flash performance on the Intel 4500 chip.

And I am on arch, too. Which Mesa version are you using? 
Have you tried this?
https://aur.archlinux.org/packages/mesa-r300-r600-radeonsi-git/