I don't think a lack of power should make it unstable...but you have flown funfly aerobatic planes before, haven't you? I did my ailerons as Tufflight suggested, but i couldn't imagine pushrods on the ailerons could cause your problems. My guess is that perhaps your elevator linkage is a little sloppy. That full flying elevator has awesome control authority, but you don't want to fly the plane too fast and even slow, if your elevator linkage has much slop at all I bet it would flutter and flex and cause the plane to get very instable.

Do think my guess could be the problem? I'd like to learn what you find...