Vertical Stringing: What am I doing? How to fix it?

[AGR416]

It's not so much vertical stringing as it is a "big group", in my opinion.

I agree with ASH556 in that you are probably not focusing hard enough on the front sight, and as such you are getting a larger group due to inconsistencies in sight picture.
Remember, the focal point should be the top of the front sight blade, not the fiber or dot.

Also, your cadence is too fast if you are working on accuracy and precision.

[Wobblie]

Lots of good ideas already. I usually try to change targets, especially the color. I've seen people put dark sight on black target and are actually not putting the pistol in the same spot each time. Maybe a small dot on white background to check?

A 3x5 card at 10 to 15 yards will focus your attention.