Y axis motors getting 'stuck' randomly

Every time I try to use the machine at some point during the carve the Y-Axis motors will fail slightly/a-lot. I’ve trouble shot this quite a bit

1: Debris in the timing chains - clean them prior and blow/vacuum them off during the carve. The same problem still occurs.

2: Loose connection on the wiring? I’ve checked everything. The wiring is good.

3: Thought maybe I’ve got a bad motor. I’ve disconnected the timing chains and run the motors individually. The both run smooth in short and long distances.

4: Hooked up all wiring with the timing chains removed, the problem still occurs.

So I’m thinking it’s something in the wiring. I swear I’ve followed the diagrams exactly. http://x-carve-instructions.inventables.com/step10/wiring-diagram2.jpg But if I run the Y-axis motors 6", they will stop and make a humming sound, similar as if they run into an object and get stuck. Seems like a short? Interference? Feedback?..

What am I not figuring out?