Blinking Monitor

First, this is an HD install.
At anywhere from 2 to 10 minutes the monitor blinks. It goes black for about 2 seconds then everything is normal with the image returning intact.
But it was doing this even from the live DVD 6.7.1
Other than this minor glitch the install has gone quite well with GrUB enabling a choice of which OS to boot.
This is an old monitor with a DVI cable, but with a 6 months old ATI Radeon graphics card.
Wth out the posts from this forum I never would have made it this far.
Thank you!