It doesn't sound like a sensored motor issue per se, just that the kv is kinda low for the voltage being used. 17.5 turn is probably a low kv motor. It just sounds like the motor is not spinning fast enough at low speeds for the ESC to get a good rotor position reading.
Increasing the start power and twiddling with the throttle curve should help a lot if this is the case. You gotta get it spinning fairly fast right off the bat, then the ESC should be able to read positions fine.
we said that...
Benjamin White
R/c Monster Team Driver
Jq the car, LST, Sportweks turmoil pro
Unconventional Techniques, Superior Results