[BrianG]

The sensors should make your starts way smoother than that at the lower rpms. I wonder if your sensors or the sensor wires are all ok? Since you have it in 'smart sense', it could be automatically reverting to sensorless only. The symptoms you describe sound like the ESC is in sensorless mode only and trying to get rotor position, but is geared rather high, so it's having a hard time at start. Once you hit higher rpms, the ESC goes into sensorless mode, so it would make sense that it runs fine then. Try 'sensored only' (not smart sense). If the motor acts funny all through the rpms now, something is wrong with the sensors/wire.