Sounds like you need to check your SMB cable from the robot to the controller. Check continuity both sides of the cable to ensure its not broken. Replace it if you have a spare. Also check the cabling going to each motor. Im not familiar with the 120 but most other models have a power and smb cable going to each motor. Any time I've seen this in the past has been due to a faulty smb cable to the controller, exposed or broken smb cable on one of the motors or exhausted rev counter batteries.
is it happening in a particular position?
Sometimes it takes a while (day or two) for the new rev-counter battery to work I find.
I've had bad SMB's do that. I have one S4C+ that used to occasionally lose rev counters after power-down - never did learn why... 
