Created attachment 141872 [details]
tarball containing daemon.log, messages, kern.log, syslog, and dmesg output

The previously described uptime test lasted (until the lockup this morning) for
9+ days, but the log files included nothing that seemed relevant.   The next
uptime test that started this morning for exactly the same graphics stack and
kernel parameters lasted only 7 hours until a lockup, and this time the
(attached) log files caught substantial error messages before the crash.  

@Michel Dänzer:  Could you please take a look at this one to see whether there
is some clue in the kernel error messages concerning the source of this
instability?