I have a Shapeoko2. Arduino Uno with Gshield. controlling Nema23 steppers. The Z axis is a belt drive Acme screw.
It has been working fine and today it is acting like it is binding. (See video).
I have done all the normal trouble shooting. Connections are good. Both mechanical and electrical. Nothing changed in any settings. Nothing is physically binding. Everything turns smooth by hand.
Sounds like you are skipping steps due to low current on your gshield. If you dial up the potentiometer it may clear this up.
Sound about right. Dial up the [pots].
A similar condition happens with the gshield when it overheats, but it sounds more like a whine rather than a vibrating pulsing sound like your video shows.
Just to confirm, when you remove the belt, you are able to turn the pulley and the z axis moves easily up and down without resistance?
Yes, the Z axis moves clean and free. No mechanical binding. I have an active cooling fan on the Gshield and the driver chips are cool to the touch.
Dial up the pot until it runs correctly 
I did turn up the pot on the Z axis a quarter turn and things seem to be cutting correctly now. The Z axis stepper is making a nasty grinding noise though.
I am getting one or two good cuts and all of a sudden the y axis goes nuts. Then the Z goes deeper. I am thinking now it is either gcode or UGS. Any opinions?
What I’m hearing sounds like a tight acme nut with no lubrication. It sounds more like a creak than a grind. What does it sound like in person?
We may need a bit more explanation of the craziness. Do you have a video?
I had a lot of problems with my gshield at first with the stock cooling fan and an overheating y axis driver.
OK, after talking with my brother in law we decided this machine needs more power. I replaced the 24v 150w psu with the 36v 400w psu I had been saving for the next upgrade. No problems so far.