Depending on the video card, it could also be a driver issue. I'm currently
setting up a mythbuntu box using the nvidia ION graphics card, and get poor
video (tearing of the video image). The answer is apparently a new restricted
driver, see here:
www.revouser.com/forum/viewtopic.php?f=3&t=237

The newest version is 190.53, which I'll try at the weekend
www.nvidia.co.uk/.../...isplay_ia32_190.53_uk.html

These links may also be useful
www.ubuntugeek.com/install-nvidia-graphics-drivers-190-42-in-ubuntu-karmi\
cjauntyintrepidhardy.html
www.revouser.com/forum/viewtopic.php?f=7&t=33