Can you take some pictures of:
1) The control board
2) The X axis motor controller on the control board
3) The wiring connectors for the x motor
a) on the control board
b) on the motor
c) Any intermediate connections/splices in between (if any)
In general, when I've seen this problem the issue I've had were:
1) Stepper motor overheating.
When it is running, how hot is the X stepper
2) Stepper motor bearing has stuff in it from a failed bearing
Swap one of the 'good' y motors with the x motor to see if it is the stepper or something else.
3) Not enough voltage going to the stepper
You've adjusted this before, maybe turn it up a bit?
4) Belts too tight
5) axis not moving smoothly, i.e. something causing the axis to freeze up.
Disconnect the belt and verify there is nothing mechanical causing it to freeze up.
6) Make sure that the wires are all good and that the connections are solid.
I've had issues on machines where the cheap insulation on the wires cracked causing exposed wires.
7) Speed and acceleration settings.
Verify that you don't have to high of acceleration settings. It's possible that the X axis is trying
It could be that the machine is trying to change x directions too fast.
Well, Those are off the top of my head.
First thing I would do is to swap a y stepper with the X stepper to rule out problems with the motor itself. Even a brand new motor can arrive with issues.