I have a helimax mx450 with an Electrify Ammo motor and helimax 25A ESC. I have only gone through about 6 batteries on it and the motor now shudders pretty hard at low throttle and then just stops. Once it stops spinning I have to lower the throttle all the way down and then increase it to get the motor to spin again. I have taken it out of the heli so it still does it with no load. Any ideas?

thanks