Here are the things I check when I find my shots stringing vertically, but good horizontal grouping.

1. Am I placing the same spot of the foreend on the rest each time, andholding it with the same tension (or not holding it at all - which is what I do)?
2. Am I pulling the butt stock back into my shoulder with the same degree of tension each time?
3. Am I resting my cheek on the stock in the same place and with the same degree of contact each time (just a light kiss - not too hard)?