His point is if the drive control loop has too high a gain, overshoot and oscillation are possible. This would demonstrate itself in a shudder or bounce, control loop instability. Lowering the gain should help, if this is the cause.
I would suggest making sure their is no mechanical cause first, i.e. verify lube oil is making it's way to both z ways as well as the ballscrew, and make sure the gibs are adjusted. You may even try loosening/tightening the gibs to see if the shudder changes any.
If all mechanical means are checked and still a shudder problem, try tuning the drive per Fadal maintenance manual
March 2003, Section 06: Axis Drive Systems, Page 191 a copy on the web here
http://www.flintmachine.com/pdfs/fad...ve_Systems.pdf
If that doesn't do it, sounds like you are on the right track with either a suspect Resolver or suspect drive in 'reading' the resolver SIN and COS input.
Good Luck.