if you turn the spindle over the "dead zone" manually, it runs, but you can hear the jitter noise.
I've recorded a video showing the problems, and it is at http://www.deligeng.f2s.com/sx3.avi
firstly, I suspected the motor had fault. However, I measured the sensor output and the coils later. It looked alright.
So, should it be the motor drive board problem?