Many, if not most, of the electrics have very thin and almost flimsy linkage to control surfaces. This is to keep things light but often, if the linkage is not installed correctly, it will flex and bend when pushed to a point where it will bend and flex.

Reading the above, two primary causes are very clear, at least to me. One is, as I mentioned, linkage. Either sloppy, loose, or lacking strength. The other is speed. And, speed will force linkage to bend, flex, or oscillate (ie. flutter).

As opjose said, this is a 3D style aircraft and if you fly it as if it were a racer, well, you're going to get flutter.. but consider the reason. The cause may be because of the "flimsy" linkage rods that bend and flex under stress, stresses caused by higher than normal speeds.

If you are not flying it faster than it was designed to fly, then consider the other reasons it may be fluttering, mainly linkage either not strong enough for that plane (if it came with that linkage then it should be appropriate for that style of aircraft) or it's just plain loose.

Check everything that has to do with linkage and make sure that there isn't even a slight bit of slop in the connections. If you have some, do your best to remove.. tighten up the connections so that any and all slop is removed. You can also get stronger linkage that is not a weight issue, or reinforce the path for the linkage by installing light-weight supports for the piano wire to pass through.

CGr.