Hi Jim,
IMO, its either in the motors, the cables, the encoders, the amplifiers or the controller
So what kind of error message are you getting ( in general terms)?
Transient errors like that are hard to nail down, but if you have already done the usual swapping of motors, cables and amplifiers, and found no pattern of error, then I'd have to start swapping boards in the controller, if it has any. Failing opto isolators give me grief like that sometimes, but I am not running your particular controller.
Sorry I can't be of specific help.
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)