https://bugs.freedesktop.org/show_bug.cgi?id=103370
--- Comment #15 from Mike Lothian mike@fireburn.co.uk --- Are there any changes when you boot the system with radeon.runpm=0, this will mean the card never powers down
What distro are you running?
You mention trying older kernel version, did you try older mesa versions too?
Can you attach your Xorg.0.log too