1. Try a newer driver or older... http://www.nvidia.com
2. Parasite removal programs.
http://reviews.cnet.com/5208-6132-0.html?forumID=32&threadID=1313&start=0 notes the five scanning tools and removal tools that are fairly safe. If one does encounter an issue, its always been damage done by the MALWARE. The most common issue being Layered Service Provider replacement which you can use LSPFIX (Hint: use http://www.google.com to find).
The thread has follow on discussions about new links if you don't want to use Google to find an item.
3. Consider if you have any P2P or network shares, that the screen may refresh at the interval that the network shares are tested. In the registry, depending on the OS, the time is usually 600 to 1800 seconds. Very interesting about your timeframe, but given you didn't note a network, I'll leave you to look into this.
PS. All the tools are free.
Every so often, maybe every 15-20 minutes, my monitor will go black for a couple of seconds. I really don't think it's my monitor. Would this be my video card doing this? I have the Nvidia Gforce4 mx420. The device manager says it's working properly. One more thing I just noticed.........the Nvidia settings icon is in my system tray. It's never been there before. When I click on the icon, it will cause my screen to go black for a second, also.