X will often crash with composite on using radeon or ati driver with Xfce 4.4.2 or 4.4.3. Vesa driver doesn't seem to have this problem. This seems to happen during some user actions, like clicking an icon, clicking buttons etc.

Kernel: 2.6.24.5-smp
Card: Radeon Mobility M7 LW [Radeon Mobility 7500]
Slackware Linux 12.1.0

End of xorg log:

Backtrace:
 0: /usr/bin/X(xf86SigHandler+0x7e) [0x80bf1ee]
 1: [0xffffe420]
 2: /usr/lib/xorg/modules//libxaa.so [0xb798aded]
 3: /usr/bin/X [0x8168704]
 4: /usr/bin/X(CompositeGlyphs+0x9a) [0x814fd8a]
 5: /usr/bin/X [0x8157648]
 6: /usr/bin/X [0x8152aa5]
 7: /usr/bin/X [0x814620e]
 8: /usr/bin/X(Dispatch+0x2bf) [0x808688f]
 9: /usr/bin/X(main+0x48b) [0x806ddab]
 10: /lib/libc.so.6(__libc_start_main+0xe0) [0xb7d5a390]
 11: /usr/bin/X(FontFileCompleteXLFD+0x20d) [0x806d121]
 
 Fatal server error:
 Caught signal 11.  Server aborting
 
 (II) AIGLX: Suspending AIGLX clients for VT switch
 disable montype: 2
 (II) RADEON(0): RADEONRestoreMemMapRegisters() :
 (II) RADEON(0):   MC_FB_LOCATION   : 0x1fff0000 0xe3ffe000
 (II) RADEON(0):   MC_AGP_LOCATION  : 0x27ff2000
 finished PLL2
 Entering Restore TV
 Restore TV PLL
 Restore TVHV
 Restore TV Restarts
 Restore Timing Tables
 Restore TV standard
 Leaving Restore TV

Let me know you need more info.