If you have concluded definately that it is OK mechanically, another point to check is the tuning parameters, do you have the parameter list? Check for comparison between Z and the other axis, They may not be identical, but usually are close.
I recently had a customer that had a similar problem on a 6M, as soon as the Z axis was moved it would fault, I checked the parameters and the Z CMR parameter had a large -value instead of positive. I am sure that no one in the plant knew where the parameter overide switch was to tamper with them, I assumed that somehow the register got overidden by sporadic noise or other means. Restoring the parameter cured the problem.
There are just a few parameters that apply to the servo on the 3M
Al.