I'm assuming you have an SM23165D motor. If it is an SM23165M motor, the sine mode commutation will still work.
The jerky motion is due to the default commutation mode, which is 6-step trap for D-type motors (MDC is the default for M-type motors). For very slow speed and smooth operation, you need to put the motor is sine mode commutation (MDS). This allows the motor to commutate using the encoder instead of the hall sensors.
Add the command MDS at the top of your program or send it from a host. It is not retained after a power cycle. The most common method is to add the MDS command in a downloaded program.
Note: The motor will not enter MDS mode until the shaft rotates one complete turn. This allows the Z-marker pulse (occurs once per revolution) location to be trapped and stored. You can return the motor to trap mode with the MDT command. You can verify that sine mode is active by checking bit2 of status word 7.
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