You were lucky surviving the first case of flutter, I would not suggest flying the airplane again until you are sure it is fixed. I am 35 years into R/C, 15 of those flying IMAC. I have NEVER seen a battery cause flutter. I have seen poor linkage setup cause flutter more times then I can count. Your previous posts are leading me to beleive that you are running ball links on plastic servo arms. On any size gasser that is a bad idea. IMO at this point you should take your aileron servos apart and inspect the gears very carefully. If all is well with the gears then replace the ball link at the servo with a metal clevis and have it no more then 3/4" out on the arm. This would be the minimum I would suggest, best case senario you would replace the servos with 150+ oz metal geared servos and metal arms.