Normally I'd say it sounds slightly spirally unstable but you say it only happens at higher speeds. That leads me to suspect the wing is flexing at higher speeds.

Try to see if it happens during climbing turns at full throttle but with the climb holding back the speed a little. Then try it at lower throttle but after coming out of a dive so the speed is high but the engine thrust low to eliminate any thrust input to the study.

I'm not familiar with the Modeltech P51. Is the wing D tube sheeted or just built up of ribs and spars? If it's D tube then I would be surprised if it's flexing but if it's open framed then this becomes a real possibility.