Thanks for the replies.
As to my flying initiating the differential roll rate, that's certainly a possibility and my be that I'm not used to flying a bipe. However, I don't see this with the other pattern and IMAC type (mono) planes I'm flying.

The center of gravity issue also has merit. I've got a fairly large battery pack in the Ulty because of the number of servos. It's situated low in the fuse and may be initiating the pendulum effect.

Nothing is ever easy!