someone's gotta know something!
Recently I've been using 'lock screen' or just leaving the pc unattended for a while, and this problem occurs there as well. So I guess it has something to do with a power management feature, although, one time it didnt respond my screensaver was running.
The weird thing is that I can't reproduce it! It happens about 1/5 times I leave the pc unattended for at least an hour, which is way too much!!