Z-axis motor slipping

I'm using 2NM cont. torque PMDC motor converte to servo and drive
with Gecko to use on my 3hp Knee Mill. I had run the mill for a few
weeks and it work ok. A few day ago, I have the problem with the z
axis cutting depth increase up to 10mm.

I had check the mechanical part.It look like no problem. The motor
look like feel hot because the load of knee is high. I'm using ratio
1:4 to drive it.

I suspect the motor slip.But it that possible a servo motor have
this type of problem since The G320 I'm using never error during the
operation.

Anyone have this kind of experiece? Thanks a lot for anyone able to
assist me on this.

Joe