Comment 30 for bug 442194

> /var/log/messages after SysRq+p and SysRq+t
This was done and is the output when the system was frozen?
On a quick look things look normal.

The currently processed code is some radeon cpu idle routine, so this would point into graphics direction, but as you stated you tried different cards, I doubt it is that (are you sure the other also froze? Better double check if you are unsure).

I am not sure what to look for next, memtest is a good candidate.
If above is the sysrq output of the frozen machine, I am going to ask someone else to have a second look, otherwise I suggest to leave this bug open for a while. Maybe you find out something, then pls let us know, if not this might even be a HW problem or very hard to find because we do not have a single pointer what could cause this yet.

You could try to blacklist the radeon drm module (not sure about the exact name, "lsmod |grep radeon" might show it).
Maybe blacklisting this one helps:
blacklist <module_name>
in /etc/modprobe.conf.local should do that.