Hopefully this helps someone identify what’s going on with their machine. Here is a picture of one of the things a loose eccentric can look like.
Machine had been working great, but over the last few jobs started acting out-of-tram. Of course, it’s always the last thing you check…I really should start with the easy stuff first 
In this case, was the lower right eccentric on the Z axis. When the tool loaded up going one direction, the whole Z axis would pivot one way, and on the return trip, pivot the other. Manifested as every other pass was a little higher than the ones in between while facing the material, by about .005"-.010" or so. Notice the “wobble” in the grain that caused, notice that it didn’t visibly impact any of the other toolpaths, just the one with the highest forces.
