This is called wing rock. It is a common problem. Doing a harrier you can sometimes increase the AOA(angle of attack) and this will help. You could also try a spoileron mix. On the parachute I only know two cures. Use the spoilerons or do them inverted. Doing them inverted putts the rudder in clean air and will cure the rock most of the time.
One other thing. Sometimes you can use the aile to stop the rock. Don't try and fight the rock all the time. Just wait for one wing to go down and bang it quickly back in place. What I mean is use a quick but heavy input of opposite aile just as the wing goes down. This will stop it most of the time. It will however on a lot of planes start right back up a little at a time.


David