you may have a poor chain line. i would move, and have moved. the cog or the chainring in or out (if you can) a couple of millimeters and see what's what. it's not always easy to see by eyeballing it. or at least that's been my experience.

i just got back from my daily ride, and after swapping cogs yesterday, had the same problem. having experienced it in the past a number of times, it was immediately apparent that i had accidentally changed the chainline. it was an easy fix.

i may also be associated with a flexing crank, chainring or even the frame's bottom bracket shell. often, heavy, or exceptionally strong riders, using somewhat flimsy cranks experience something similar.