Hmmm. I haven't had that problem. I did the normal 1/4" blue cor with a top and bottom piece. Somehow that hollow shape along with the spar and some bamboo skewers made mine very rigid. My power system gets my plane to around 65 mph in level flight and up to 80 in a dive. Never had a flutter.
It's weird how the 1/4" foam is so flexible but when you put 2 pieces together with a spar in the middle, it becomes quite rigid. Must be an engineering principle in there somewhere...
Maybe solid foam allows for a lot more flex?
I also built an "Ultron" 3D plane that's a full fuse design. Amazingly strong and rigid design. Love the way it flies. http://www.rcgroups.com/forums/showt...ghlight=Ultron