I would say that either you're getting significant velocity variations shot-to-shot, or it's a most likely a breathing problem. If you're not carefully measuring your powder, or your seating pressure is very inconsistant, you could see velocity spreads and vertical stringing. If you load the rifle well, then breathing is the next potential cause I'd look at if I were coaching you. If you are supporting the rifle with your shoulder, then shooting at different times during your natural breath cycle can cause vertical stringing. To fix this you need to breath naturally, then after your last natural exhalation you fire during your natural respiratory pause. Don't hold your breath beyond when you're body naturally wants to breath or you'll harm your shot, too. Just get in the habit of timing the shot with the normal pause between breaths, if you don't get the shot off in the pause, go ahead and breath and try for the next one. If you do it right, and don't force it, it'll help out if that's the problem.

Mike