Mike,
My first Falcon exhibited the same problems. The solution was in the elevon set up. I slaved the rudder channel to the aileron and did not set equal amounts of movement in them (by accident). Consequently the left aileron (rudder) was moving a lot more than the right. This was also true when the elevons were acting as elevators. You might want to check the ATV on your rudder, and see if it matches your ailerons. Also you might need to check expo for both channels.

Good luck, Lou.