I would tend to think of too much backlash compensation in a setting as well, except that such a problem would not 'just happen today' unless you were 'fiddling with parameters just today' ![Big Grin](images/smilies/biggrin.png)
I would carefully check any couplings between motor and encoder or resolver for slackened screws. It could be that a little bit of lost motion in such a coupling could cause the controller to jerk the motor a little bit when it attempts a reversal but does not see the expected reponse in the position feedback loop.
First you get good, then you get fast. Then grouchiness sets in.
(Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)