Well, in that case.......

Thud_Driver, I bow to you oh Great Omniprescient One....

DR, did you ever do anything about the wagon being way too nose heavy? If not then we may be chasing a non issue about the wingtips. Granted the tips may have added to the issue but it may be a bit hard to figure things out at this point.

Some models are just murder. I tried helping a father and son with a Piper Cub ARF one time. That model was the most spin prone POS that I have ever had the misfortune to fly. Balance and washout looked good but it would just stall and drop a tip into a terrible spin at the drop of a hat. Even from level flight unless the speed was somewhere around Mach 1.5.