The only way, as far as I'm aware, that the rotor of a brushless motor can diminish in strength is if it's been overheated. Overheating can easily cause the rotor to demagnetise, but other that that...they should never lose strength. Pretty much all RC-specific brushless motors use Neodymium as their rotor material, which is a permanent rare earth magnet and should never lose strength on its own, unlike those in most brushed motors.
If your Peak motor appears to have been losing strength, then you've probably either overheated it a little at some point, or perhaps your cells are going a bit soft? Then again, if it was "notchy" when you first got it, then this points to the motor having been overheated.
