I read this last night and moved on to think about it and completely forgot about it. That's my bad. This issue could be a severely sagging battery, a bad connection that cant' withstand the current, a bad FET on the control board or a bad motor. You can start by putting a voltage tester on the battery leads and spooling it up. this will tell you if in fact the battery is sagging. If the FET is bad, you will notice either a crack, sweating (looks wet) or both. The FET if you dont' know will be the largest black block on the control board. If the motor is bad then the windings will be shorted together. You can use an OHM tester and ohm out each of the wires if there are 3. They should all be the same. You can also take two of the 3 and connect them together and spin the motor. This will cause a braking effect which will make it harder to turn. If one set of wires has a weaker brake than the other ones, you found the issue. Also, connecting all 3 wires together will cause it to have full brake. If all 3 together is no stronger than just two of them, then you have a bad winding. Again, this is for a brushless motor.
Funny thing is I thought when they went bad, they swelled up. But these aren't.