Intermittent FPS lag dual boxing.

So I upgraded my hardware sucessfully, and now I can run 2 accounts on a single Phenom II X4 940 B.E , 8GB ram, 4890 1gb. Using Keyclone, I can seamlessly use dual displays with ease. But not everything is perfect. I hope it could be though, with your help. Using Vista 64

I can run 2 accounts at 1680x1050 using the above machine at the least 100+ FPS using everything maxed except shadows (i dont care much). Which is awesome. BUT , when running two accounts, I intermittently get this FPS lag - more like an FPS 'dip' every 10 seconds or so to around 20FPS that lasts around 10 seconds each time. Needless to say that is pretty dang annoying. Everything else is fine even in cities though I haven't checked any wotlk zones since my chars aren't high enough yet. I tried to tinker with the display settings, turning off sound, etc, but the problem is still there. I even enabled write/caching, disabled the page file in windows, but nothing to fix it.

What could this be due to?

As of right now I'm downloading Windows 7 RC to see if that would change anything. But I wanted input from the pros here.