Could be a number of things. Does it happens when just playing one WoW? Does it happens with other apps or 3D games?

Here is a list of things to do:
1. Check your memory - google for mem86 and use that program
2. Do a full disk scan, schedule to do it at start-up including a sector scan
3. Clean up old your hard-drive - go to the properties and click the clean up button
4. Defragment you hard-drive
5. Update all your drivers - chipset, video, sound, network ...
6. I possible swap the video card out for testing purposes, maybe ask a friend if you don't have an extra video card. Perhaps even try your current video card on a different computer

Items 1-5 are basic maintenance steps which may fix a number of issues, or not ... but it should hurt

if you done everything and still unable to find where the issue is, then back up all your data, reformat the drive and re-install everything. It will take less time then to figure out what is going on, plus you will have a fresh system :-)