Hi Helmut,
Since your Xorg.log ends with "Backtrace", I guess your server is
actually crashing when the human interface freezes. It would be very
nice to catch this full log up to the end of the backtrace.
If you have a ssh access to the machine, could you try attaching gdb to
the X server *through ssh* before the crash with
gdb -p $(pidof X)
and see if you can get a backtrace (with bt full)?
Brice