I have just unplugged the sensor wire and turned my GTB on - Same symptoms - My money is on the sensor wire is damaged, (like the above posts from- pugs and emtee). Change the sensor wire and that will most likely resolve it.
I notice you switched the motor. What motor was it - i.e. did it have a seperate sensor lead or was it soldered into the motor? If the former, my next question is - did you change the sensor lead?. If it it's the latter, did you clean the contacts on the ESC/ check the solder tabs etc.