Has anybody out there used a servo motor as a spindle, and used the step/dir spindle outputs from Mach 3 to control it?

I'm using an A-B servo with an Ultra 3000 to control it. The Ultra is set to follower mode and accepts the step/dir inputs from the computer.

I need some help! My servo motor keeps faulting out with either a thermal overload or an encoder following error. I also get some really bad vibrations at certain RPM. It gets so bad sometimes that the lights in my basement will flicker at the same "vibration frequency".

Running at a very slow RPM shows that the servo is in fact acting just like a stepper motor (due to the step inputs) and is not a smooth rotation. So my guess is that the servo motor is working way too hard to do the start-stop-start-stop stepping at high RPM and thus causing the faults and vibrations. I messed with the counts/rev in Mach 3 and the follower ratio in Ultraware but could not find a combination that runs the motor smoothly.

Is anybody successfully doing what I'm trying to do?????
Any ideas or tips????

Thanks!