Look on the drive for a potentiometer labeled BAL. This is the balance POT and can be used to null out the .7mV offset you are seeing at the drive.
Watch the axis pulley while adjusting the POT. Adjust the POT slowly one way until the pulley stops moving. If the pulley speeds up while adjusting then turn the POT the other way. Again, adjust the POT slowly or the axis may run away.
If you cannot see the axis pulley that is drifting while trying to adjust the POT then have someone else watch the pulley while you adjust.
If the POT will not stabilize the motor, or if it is worse after adjusting, then the POT itself may be bad and will need to be replaced. The wiper in the multi-turn POTs tend to go bad over time.