I have this problem for a long time, today I decided to kill windows, and put a completely fresh setup. But the problem still persist.
I believe the problem lies in GPU drivers, not sure whatever it is an Intel or Nvidia, but one of these is a problem for me.
So, when I installed fresh windows the sleep option was dimmed, so I couldn't get to sleep, as I installed GPU drivers, sleep option was enabled, but than there is the problem described above. No, it's not a windows issue, same problem is with Windows 8!
Looks like "System Interupts" is using 18% CPU - whatever that is, it's using 1 core.