Hmmm.... It doesn't happen on each shot, huh? I suggest next time this happens to mark the arrow that does it. Check to see if it's the same arrow each time.

Otherwise, I'd say your arrows are seriously underspined. Or your brace height is too low. Or your nocks fit too tight. Low brace and nocks that are too tight will definitely interfere with paradox and overspine arrows always contact the belly side of the riser.