If your controllers are identical models, and one is acting up, I would suspect some kind of hardware malfunction, such as a failing circuit board. I have no idea what your controller looks like inside, though, so you'd best get a real service technician on the phone for help.
If the controller tells you that there is an error, is this just "after the fact" when it does an in-position check when the arc is completed, and then attempts to correct it, or does it halt with an error before it does the faulty arc?
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)