I think the problem may be in the last line below.
RS BO 5, knit 1 more st, work in patt as set to last 7, knit to end. WS, BO 5 sts. p6, k1 PMwork in patt as set to last 8 sts, PM, k1 purl to end. That was fine. It's the next part that messes me up. Dec 1 st at each edge every RS row 3 times as foll: ssk, knit to 1 st before marker, p1, work in patt as set to marker, p1, knit to last 2 sts, k2tog. Next row (WS) purl to marker, remove marker, p1,k1,PM, work in patt as set to last 7 sts, PM, k1, purl to end removing original marker.
It seems to me that you start with 8sts on the ends outside the markers. Then you decrease one st at each end on the RS, leaving you with 7sts outside the markers on each end. On the WS row you purl those 7sts and move the marker over 2sts and work to the last 7sts and are told to PM, but the marker is already there and there's no marker to remove a couple of sts later. It probably should be work to the last 9sts, PM,k1 purl to the end removing the original marker.
The pattern in the main part between the markers may be the reason that you need the markers to begin with and need to keep moving them to keep that pattern going as you decrease at the ends.