Motor flutters at W.O.T.

Hi, my scooter (Schwinn S350) has developed a flutter only at full throttle. It has ran great until lately. I have a shunt mod, as well as overvolt from 24 to 36 volts (motor only ESC is 36 volts). I think it would be the batteries getting tired, but I can't afford to guess on what parts to replace. The scooter runs great until w.o.t. Any help would be great, thanks.