Hi, Sounds like an X issue. It might be helpful to post your /etc/X11/XFree86-4 config. Monitor section.

And maybe manually editing your x configuration doesn't work in 3.6 I don't know that for sure, but since this is debian you can open a terminal
and, as root, type "dpkg-reconfigure -plow xserver-xfree86" press enter and you can go through the program to setup X. This will let you choose you video card also.
Good luck. Let us know how you make out.