The spindle on my VF3 stalls out every now and then, right in the middle of running a program. It doesn't do it all the time, but occasionally it does, and breaks the tool.
The first reason would be because it's being driven too hard.
Next would be loose connections. Make sure you have no dead batteries that will cause you grief if the power fails, then disconnect all power. Wait five minutes and tighten every screw holding a wire.
Next would be a failing position sensor for the motor.
Followed by a failing position cable/connector.
The servo drive could be failing.
Check any error codes there should be one if the drive is having issues.