I'm only guessing but I would imagine QC is considered low priority so if any othet tasks are running it will hold off running for a while.
I've never heard of it running unscheduled though.
You might want to open a case with Technical Support on that issue as it could be an as yet undiscovered bug.
It's free by phone or online chat and linked under Useful Links at the top of this page.
Private Message conversation with mqsuser
Problem still exists on XP PC W7 PC seems OK
Then I would contact Technical Support as I suggested.