Need a bit of advice here.. I have 3 "Japan Servo" Stepper motors with probostep drivers running a foam cutting table with a dremel router

KP56LM2-097 from JAPAN SERVO
Parameters:
Unipolar / 2 phases 6 Volts DC (nom) 1.2 Amps / Phase (nom) 2.4 Amps / Motor 1.8 Degree/Step

Each motor is driven by an independant power brick 18v @ 4.9amp the probostep driver are set to limit current to just at 1 amp (much higher and they get real hot)
they seem to run fine at higher speeds (cutting a straight line) but seem to chatter a bit when cutting a curved line.

I can duplicate this by running them individually from the MDI tab, when i slow the speed to about 1.26 to 1.3 they chatter in the same way when cutting parts. This is throwing the cuts WAY off. 

Above or below that they work fine.

Cna anyone sghed some light on this?

I didn't realize the lack of info that i put here..
Here is a little more detail.

I can duplicate this with all 3 axis using the same procedure
On the MDI tab I enter g1 x20 f5 to start the movement at a feed rate of 5 (not sure I need the G1 in there but no matter).

As the axis is moving, I hit the override button to slow it down. As the axis gets to 1.3 the motor starts to reverse itself every couple of steps, I continue to drop the speed and at 1.2, the reversing stops and the motor continues to move along without issue.
I then start to increase the speed and at 1.26 the reversing every few steps process starts all over again, As the speed goes over 1.3, the movement gets smooth again and the issue goes away.

Sounds like it could be resonance, do your drives have adjusters for it?
Hood

Looks like it was the fact that my drivers were in full step mode.
I have now heard that this is common when running steppers at a slow speed and in full step.
I changed to 1/8 step and it seems to have clear up the issue.

Yes that is the reason you have microstepping, it is to smooth the motion, especially at low speeds.
Hood