Back pedal the bike...slowly...while watching the rear derailer. If you have a stiff link, the derailer will jump slightly as the link passes over the jockey pulleys or you may be able to see the link as it passes the jockey pulleys. It will stand up and away from the pulley.

While you are doing this check, watch the chain as it passes over the small ring in front. The ring should be very straight. If you see a wobble, that is likely your culprit. You may also have the low limit on the derailer set too far inboard so that the derailer can nudge it off the ring as the frame flexes during hard efforts.

If all else fails, you can add a chain watcher to your bike. They look like this



Not real elegant but it does work.