Hello,
If you have a problem with 2.6.29 and no write-combining entry in
/proc/mtrr, could you try replacing libpciaccess0 with the one available at:
http://people.debian.org/~bgoglin/mtrr/
It contains a tentative fix that may bring the WC MTRR back.
You'll have to restart X after installing the new package. Please send
the resulting /proc/mtrr and let us know if it helped.
Brice