Flutter is a killer, and airspeed is the cause.

The cure is mass balance. I once had a 60 size sport model and at full power in level flight it was fine.
At full power in a slight dive it suffered aileron flutter.
I rebuilt it and added mass balances, after which I could perform a steep dive at full power without a problem, and I have mass balanced anything fast ever since.

My next prop model is a 30% yak with a 55cc, and I guess I'd better mass balance even though the instructions don't mention it.

As an aside, most of these big light scale aerobats are flown with quite fine props, and that will limit their speed somewhat. Maybe the 20x10 was a contributor. A 22/8 might have contained the speed just enough.