Limits on small motor moves?

I'm using SM1720D85C-DE in a current design. Due to the hardware design and customer's specs I need to be able to make numerous 'small' (~15-40 motor counts) moves in the same direction. I've adjusted the position error down to 15 (E=15 F) and tried PID settings both 'tight' and 'loose' (defined/calculated by SSI application). I'm seeing what I believe is odd behaviour. For the most part everything works as expected BUT at seemingly random intervals the motor will 'jump' forward/backward by ~350ish motor counts. Any thoughts?
Thanks in advance.

JT