Some times vertical stringing can be cured with a powder increase or decrease. Load some up in increments of .5 grains. Start low and work your way up. You may find that the vertical stinging is cured when you find a sweet spot. If you have a chronograph see if your SD is large and velocity has a wide variation. You did not mention if the bullets walked vertically (one shot over the next over the next) or were random (real high, real low, then some thing in between). If the gun and rest are taken out of the equation, and I shoot vertically, I look at loads, if horizontal, I look at me or a stiff cross wind. Of course that is just a general rule of thumb and can not apply to every situation. Shotgun patterns usually means something is amiss with the gun/scope - generally. Like I said that is not a hard fast rule, but some thing to consider. It can be a challenge sometimes to find the sweet spot, but I like the load development phase. Good Luck and Good Shooting