I would suspect that the drive is not getting feedback from the tacho on the motor.
You can make the P in your tuning 0 and only use Ff1 so that linuxcnc doesn't correct the position loop and see if the motor wants to run away. That normally means that there is tacho issues.
Be carefull though. It can run away and crash. Make sure that the following error will disable the drive. Better to take the motor or belt off before doing this.
