If I hold down any key on my keyboard, once every second it will freeze for a sec, then keep going. Same thing with my mouse... if I move my mouse around it freezes once every second and at the SAME TIME as my keyboard. Mouse is wireless, keyboard isn't. This happens both under load and while idle. I tried with both the USB 3.0 ports in the front and the regular USB 2.0 ports in the back, both have the same problem. When I turn my computer on/restart its fine for a while, then it randomly starts happening anywhere from a few minutes to a few hours later.
This isn't much help, but I have the same issue with gtx480 sli. With sli disabled the issue goes away. I've read some things that say it has to do with keyboard/mouse polling rates, but haven't had much luck with that angle.