Please note that when the freeze occurs, most of the system is alive and well, only the X server is wedged.  It is possible to ssh into the system for debugging purposes.

When I attach to the X server when it is frozen, I see one of the following stack traces:

#0  0x00007fa8715e4cd7 in ioctl () from /lib/libc.so.6
#1  0x00007fa86f5523bd in drm_intel_gem_bo_start_gtt_access () from /usr/lib/libdrm_intel.so.1
#2  0x00007fa85e943241 in intelFinish () from /usr/lib/dri/i965_dri.so
#3  0x00007fa870251ac6 in __glXDisp_SwapBuffers (cl=0x503ff00, pc=<value optimized out>) at ../../glx/glxcmds.c:1425
#4  0x00007fa870254de2 in __glXDispatch (client=0x2065b00) at ../../glx/glxext.c:523
#5  0x000000000044e304 in Dispatch () at ../../dix/dispatch.c:437
#6  0x0000000000433d8d in main (argc=10, argv=0x7fff7ba0b9e8, envp=<value optimized out>) at ../../dix/main.c:397

or

#0  0x00007f8ff1befcd7 in ioctl () from /lib/libc.so.6
#1  0x00007f8ff020d343 in drmIoctl () from /usr/lib/libdrm.so.2
#2  0x00007f8ff020d5cb in drmCommandWrite () from /usr/lib/libdrm.so.2
#3  0x00007f8fefd7f528 in I830Sync (pScrn=0x9950a0) at ../../src/i830_accel.c:214
#4  0x00007f8fef522e6c in exaWaitSync (pScreen=0x9ca4e0) at ../../exa/exa.c:1065
#5  0x00007f8fef5240a1 in ExaDoPrepareAccess (pDrawable=0x5f89630, index=0) at ../../exa/exa.c:509
#6  0x00007f8fef528d99 in exaCopyDirty (migrate=0x7ffffc015458, pValidDst=0x168853d0, pValidSrc=<value optimized out>, 
    transfer=0, 
    fallback_src=0x5f89670 "��������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������"..., 
    fallback_dst=0x7f8fed628880 "", fallback_srcpitch=32, fallback_dstpitch=64, fallback_index=0, 
    sync=0x7f8fef522e80 <exaMarkSync>) at ../../exa/exa_migration.c:218
#7  0x00007f8fef52927d in exaDoMoveInPixmap (migrate=0x7ffffc015458) at ../../exa/exa_migration.c:274
#8  0x00007f8fef529a6f in exaDoMigration (pixmaps=0x7ffffc015440, npixmaps=2, can_accel=1) at ../../exa/exa_migration.c:683
#9  0x00007f8fef52af3c in exaTryDriverComposite (op=3 '\003', pSrc=0x5f89930, pMask=0x0, pDst=0x61bffc0, xSrc=0, ySrc=0, 
    xMask=<value optimized out>, yMask=<value optimized out>, xDst=<value optimized out>, yDst=<value optimized out>, 
    width=<value optimized out>, height=<value optimized out>) at ../../exa/exa_render.c:642
#10 0x00007f8fef52b665 in exaComposite (op=3 '\003', pSrc=0x5f89930, pMask=0x0, pDst=0x61bffc0, xSrc=0, ySrc=0, xMask=0, 
    yMask=0, xDst=0, yDst=0, width=17, height=17) at ../../exa/exa_render.c:935
#11 0x0000000000539f78 in damageComposite (op=11 '\v', pSrc=0x5f89930, pMask=0x0, pDst=0x61bffc0, xSrc=0, ySrc=0, xMask=-4, 
    yMask=<value optimized out>, xDst=<value optimized out>, yDst=<value optimized out>, width=<value optimized out>, 
    height=<value optimized out>) at ../../../miext/damage/damage.c:643
#12 0x00000000005325da in ProcRenderComposite (client=0x71dbe50) at ../../render/render.c:720
#13 0x000000000044e304 in Dispatch () at ../../dix/dispatch.c:437
#14 0x0000000000433d8d in main (argc=10, argv=0x7ffffc015958, envp=<value optimized out>) at ../../dix/main.c:397