First, are you sure it's your nock and not your string? Second, if you're doing it only 10% of the time, that indicates that something is slipping in your form 10% of the time. You might try some close up, blank bale work with your eyes closed, focusing on your anchor point to see if you can determine what you're doing differently when you hit your nose. Another possibility is the use of a vidcam or a knowledgeable observer to help you detect the "occasional" break down in your form.
I've had similar types of episodes, and it almost always had to do with a slight change in my anchor point, and/or, a difference in the way I held my head.