Hi!
Flutter is due to a too weak aileron set-up and too much speed !
Check your ailerons so that they are stiff (the balsa that is) and then check the linkage. It's vital for any set-up that you adhere to the general law that says: shortest possible servo arm and longest possible aileron/rudder arm and no flex!! That's very important!!!! Othervise you build in flex in the system!

Then ...the Moki engine is just too big and powerful for that plane...that's the main problem! Too much speed! Use a .91-1.20 fourstroke and everything will be fine.