Check if the plug on the backpanel from the motorcable is firmly plugged in and wiggle the plug when turret is under power
if there is a loose connection between plug and socket.
Check / measure the cable from plug to the motor for breakage.
Unscrew the motor and try it with 12V if it runs uniformly, perhaps the brushes are worn out.
Plug the cable back and measure if the DNC-board provides the 12V and check the 12V input from connector X63 on DNC and X13 on power supply.
These connectors are known for bad connection due to oxidation. Be careful when testing, the DNC-board, motor controller etc.
are also connected to mains power!
Check the mechanic for free running.