Random occurences like this are sometimes difficult to diagnose. Often the best way is to go through a process of elimination.
So here are some suggestions:
1. Update your AV and SpyWare scanners. Then do an indepth scan (NOT the quick-scan), firstly with the AV scanner and then the SpyWare. You should use at least 3 SpyWare scanners (e.g. Ad-Aware, Windows Defender, Search & Destroy).
2. Disconnect the PC from the mains. Attach a grounding strap to your wrist and to a metal part of the PC. Blow or vacuum out any dust. Reseat all of the following: RAM, Video Card, Sound Card, "other" cards, all cables, all connectors.
3. Check that the video cable running from your PC to the monitor is firmly connected at both ends and is not damaged.
4. Ensure that the video cable and the power cable to the monitor DON'T run parallel to each other. (Even though the video cable is screened it is still not advisable to run a power cable in parallel. Electro-magnetic fields don't always behave like the text books say they should!!)
5. If the problem persists try to determine if you are doing something specific when it happens.
Let us have your feedback :)