Honestly, it sounds mostly like shooter-induced error. It is hard to imagine the load you describe being so slow that you can get velocity-induced stringing just going from 100 to 150 yds. Maybe at 250 or 300, but not 150.

Do you have much experience shooting that for with centerfire or anything?

If we take that out of the equation, vertical stringing is usually caused by variations in velocity.
1) Do you consistently tap your powder and measure it exactly the same each time?
2) Do you try to apply even pressure each time you seat a bullet?
3)Is the sabot snug, not loose or hammer-down tight?

Good luck