JDAn, another area that is generally overlooked by most is servos! I have had this happen and have seen it happen to others. When using a standard servos on a large aileron the gears can wear down, or the servo strength can decay from age. This can be especially true if you are doing some really hard aerobatic flights. The servo is only a small electric motor that moves gears to turn the linkage arm. You may want to check your servos to see if they are still producing enough torque. This can be quickly and easily done. Turn everything except the engine on. Turn the ailerons and see if they can be moved easily with hand pressure. If they can be easily moved, you may want to invest in new, or stronger servos. Now we are talking about doing this by feel so the amount of pressure you feel is right is totally up to you.

If the servos are still strong, I agree with the others, it is probably flutter.

Thanks,

Reg