Ok, this is certainly a kernel issue. After changing over to Fedora's 2.6.29.1-111 kernel, the issue has gone away.
Here are related kernel log entries when the problem was occurring with the ubuntu kernel:
[ 30.413499] radeonfb 0000:00:10.0: Invalid ROM contents
[ 30.593004] [drm] Initialized radeon 1.29.0 20080528 on minor 0
[ 31.163190] radeonfb 0000:00:10.0: putting AGP V2 device into 4x mode
[ 31.461184] [drm:radeon_do_init_cp] *ERROR* could not find ioremap agp regions!
Ok, this is certainly a kernel issue. After changing over to Fedora's 2.6.29.1-111 kernel, the issue has gone away.
Here are related kernel log entries when the problem was occurring with the ubuntu kernel: do_init_ cp] *ERROR* could not find ioremap agp regions!
[ 30.413499] radeonfb 0000:00:10.0: Invalid ROM contents
[ 30.593004] [drm] Initialized radeon 1.29.0 20080528 on minor 0
[ 31.163190] radeonfb 0000:00:10.0: putting AGP V2 device into 4x mode
[ 31.461184] [drm:radeon_