I might have had this same problem, but it only happened once....

In my case, it was the result of a frozen process or something. Compare WCG threads in ps -e and top; there should be one or more which are in ps -e but not using CPU in top. Kill those and they will restart/resume. Assuming it's the same problem, of course.

If it's happening repeatedly, there might be a hardware problem. (overclocking, bad RAM?)

EDIT: I'm tripping for certain. Top is already identifying the bad process; just kill the one/s with the <defunct> tag. They'll probably show up that was in ps -e, too.