Increases of the waist should be made every 10th row, not every 8th row.
Instructions for BACK should read:
…Next (inc) row (RS) Selv st, k1, KFB, knit to last 4 sts, KFB, k2, slev st. Repat inc row every 10th row 4 times more - 84 (94, 104, 114) sts….
Instructions for FRONT should read:
Shape Waist and Armholes
…2nd row after 2nd increase (RS) = 12th row of stripe pattern…