Check that all the magnets are still firmly glued inside the bell. Also turn the motor by hand - are the bearings still smooth? Is it running hot, can you keep your hands on the motor after a flight or not (probably OK since you say "no extra heat")?

I know about a case where this particular motor has thrown the magnets (was running hot) and another case where the bearings gave up after 50 flights.

Can you also confirm that the motor + ESC combination is the same and worked before. Did you change the timing advance settings on the ESC by accident? This could cause the motor to go off sync at higher power settings.