I had exact problem on 6700 non-xt few months back. It's strange issue and I never got to bottom of what worked. but one of following fixed the issue.
1. turn off monitor and CPU. (Not soft off but physical sw off)
2. reboot without connecting monitor, twice pr more times.
3. update driver to latest.
Hope the fault clears. Next, I Was going to try removing cmos battery and bios reset next.