Ive heard of this and some have said that sometimes the oil/grease from the bearing goes on the sensor pickup so it cant read the starting point.Give it a clean with a electric cleaner or CO2.
Also heard that some motors have a bad solder joint on the back board so that might be a option too.
casterracing EX-1 Pro, SP Silver Arrow
TOP Racing Scythe, SP 17.5, SP GT Pro 2.0
TOP Racing Scythe, SP 4.5 V2, Hobbywing Xerun 120SD 2.0
TOP Racing Scythe, SP 13.5, Hobbywing Xerun 120SD 2.0