Freezing with Radeon HD 5670

I've got a pair of Radeon HD 5670 graphics cards and an AMD CPU. When I boot, "desktop 3.3.8-2.mga2" works fine, but any newer kernel fails to bring up a gui. Instead, it hangs while looping through:
If I add the parameter "nomodeset" or "radeon.modeset=0", it will give me a desktop, like normal. And in safemode, it will drop me to a command line.
However, after a varying amount of time, the video will freeze, and will not respond to any input. It usually does not take very long, although it appears to be associated with mouse movement (if I don't move the mouse, e.g. video will keep playing). After the freeze, audio will keep playing, so I'm convinced it's a graphics problem.
failed to allocate
size: 8294400 bytes
alignment 0 bytes
domains: 2
If I add the parameter "nomodeset" or "radeon.modeset=0", it will give me a desktop, like normal. And in safemode, it will drop me to a command line.
However, after a varying amount of time, the video will freeze, and will not respond to any input. It usually does not take very long, although it appears to be associated with mouse movement (if I don't move the mouse, e.g. video will keep playing). After the freeze, audio will keep playing, so I'm convinced it's a graphics problem.