novak 6.5 motor issue

[What's_nitro?]

I think if it were mechanical (a bearing, bad rotor, etc.) it would be a constant problem. I'd check the sensor harness and make sure it's clean, and fully inserted. Has the motor ever taken a hard hit? The hall sensor array in the back may be loose, and is sending strange signals to the ESC that it can't respond to. You can remove the back of the motor (3 screws) to expose the hall sensors. Make sure the circuit board isn't loose or dirty. Blast it with compressed air if you can. Beyond that, definitely look into a more powerful setup.