Thanks for the replies guys. I've got the weekend to myself which happens pretty much never so I would really like to get this fixed.

The plot thickens: I've figured out that it's not combat that triggers the lag, but is a direct result of how many keys I am pressing and how many windows those presses are being broadcast to. I enter combat and start spamming my dps and heal keys, that's what triggers it. I can get a similar result if I simply spam the crap out of jump, although not as severe. I can get the mouse to jump a little with only one window open simply by holding down any button. However, it only happens with keyclone open. So I guess Keyclone becomes the target once again.

I have disabled all of my addons, cleaned out my case, rolled back my overclocking, stuck an extra fan in there, reinstalled keyclone, tried all kinds of priority settings and it still won't go away. Next up is taking out 2 of 4 memory sticks. This is really frustrating as I thought the problem fixed itself a few days ago, everything was working fine last night and I turn on my comp this afternoon with my gf out of town and it's all gone to hell.