i haven't seen this type of thing before and am not sure if this is bug report material or lack of understanding on my part -- and if bug report, against what, exactly?
Try disabling Radeon DPM with kernel switch. I don't remember the exact name, but it's probably something like radeon.dpm=0. I have recently read on the channel that DPM is enabled by default as of 3.13. If it's not DPM to blame I suggest to ask the guys on #radeon directly.
-- Kind regards, Damian Nowak StratusHost www.AtlasHost.eu
