Hi, I have experienced this with a couple of customers PC's running Catalyst drivers. The issue was to do with mixed driver versions due to old driver files being left around when new drivers were installed. This happens quite a lot with video card drivers because the old driver files are rarely got rid of completely by normal means so you end up with certain system calls picking up a mix of files from different versions of the driver. This usually doesnt cause big issues, but it is known to effect performance sometimes and upset some games.
Ok, on to business, assuming this is the issue, you need to completely remove the installed Catalyst drivers. If you click on this rather useful link
http://www.tweakguides.com/ATICAT_1.html
on page 4 is a comprehensive set of instructions for removing the drivers properly, as you will see it is more involved than just uninstalling them.
If this doesnt work (but its worth doing anyway) the next thing I would try is to remove and reseat your card and to also diconnect and then reconnect the power connector for the card (ok, you might try this before mucking about cleaning up drivers).
One other thing if this still does not resolve this issue, try using the other video output on your card, its possible one of the outputs has an intermittent fault (tho this is pretty rare).