ATI X1900's in Crossfire: graphical issue

I just slapped together some hardware i managed to scavenge together, the system specs are as such:

4gb DDR400 RAM
3 Hard drives (two 150s and one 120)[Not raided]
dual ATI X1900 video cards in crossfire (newest drivers installed)
Windows XP SP2 with all currently availible updates
AMD X2 4600+ Processor
Edit: Motherboard is ASUS A8R-MVP (with updated BIOS via the ASUSupdater)

The problem I'm having is this:

I like to play my games windowed so that I can still run other applications in the background and easily switch to them. Now, when running a windowed 3D application (It's World of Warcraft), I get a very strange glitch. If I highlight anything in the taskbar for more than a second or so (when the tooltip comes up) the World of Warcraft window slows to an unbearable pace, something like 2fps. I also run trillian (an instant messaging program) that has window transparency enabled, and when there's a transparent window on screen while World of Warcraft is open, I get the same problem. If I have 2 transparent windows open when WoW is open, the entire system slows down until I minimize or close the transparent windows.

I've messed around with the video settings but can't seem to figure out what the problem is. This never happened to me with my old graphics card (which was a GeForce 6800GT). Can anyone offer any insight or a possible fix to this problem? I'd really appreciate it.

Thank you.