Depending on where you put the camera there could be a a lot of aerodynamic disturbance. It seemed to be mounted on the wing root near the fuselage on the left side. The model seemed unstable and not wanting to settle into stabilized flight, and you said you retrimmed the model. This indicates where you put the camera was a bad spot, wish you had provided a picture of the camera and it's mounting location, I'd be interested to know.
Gee Bee's are pretty slick airplanes designed to be the exact shape that they are to be flown relatively fast. Sticking things out in the slipstream, especially near the wing root on a critical airplane like a racing plane and especially a Scale model of a Gee Bee, one could expect trouble! Interestngly the P-38 Lightning had had airframe vibrations and instability that was traced to the intersection fairing of the wng root and the pilot pod causing disturbed airflow over the stab. Your problem was something like this, it seems only much worse. Yours affected controllability.
I have a Nitroplanes 71 inch Gee Bee and am looking forward to flying it soon after it is refinished.
Chris...