Agree with Ingrid that 2 garter sts isn't enough. The width of your border sts has to be heavy enough to offset the tendency to roll by st st. On a 6" scarf, it's not uncommon to have 1+" of garter on each side.
You could try crocheting a border after the fact to add fabric on the offending edges. What fiber are you using? Blocking may or may not help a little....
Keeping to the knitting basics