If you start with 80 sts and as you say knit 8 and k2tog around you'll wind up with 72 sts (8 decreases). On the second round, knit these 72 (that is, knit plain). On the next decrease round, consider it 8 sets of 9 sts and k7, k 2tog ending up with 64 sts (8 decreases). If the next row is knit plain, just knit these 64.
You don't want to go back to 80 sts, you want to keep decreasing the number of sts on the needles. I hope this helps?