Sounds like something is disabling the Y axis servo. I can think of two possible reasons:

1) The Fanuc 11M has analog (3-phase full wave) servos, and there are 3 fuses on the back of the servo amplifier. In addition to the 3 large "cartridge" fuses, there are 3 smaller "Indicator" fuses wired in parallel with the big fuses. If a big fuse blows, the little fuse is supposed to blow with it, shutting down the servo. The little fuses have little white flags inside that pop up when they blow. Sometimes, these flags don't pop up, so the servo can fail to shut down correctly. Check the big cartridge fuses with an ohm meter, and look for an indicator fuse that's "popped up".

2) This is my best guess: Each axis would have DOUBLE limit switches in both directions. If the axis moves too close to the end of it's travel, it hits a regular OT (over travel) switch which throws an OT alarm on the display. If the axis PASSES that OT switch, it hits an "Extreme OT" switch, which basically shuts down the servo. If your Extreme OT switch is stuck on (in either + or - direction, then the servo can't run. That causes the main servo MCC relay to drop off immediately, causing the axis to drop a bit, and then it applies the brake in the motor. Vertical axes that can "fall" if the servo doesn't start usually has an electric brake in the motor so the axis can't fall very far.

Check those extreme OT switches to see if one is stuck on. Also, be aware that these switches are wired "Normally closed", so that a broken wire or loose connection can act like a switch that's stuck. If the wires to those switches go through a flexible cable duct, you might have a broken wire inside the duct. That's a difficult thing to find.

Are you getting any alarms on the CNC when this happens?

Dan Fritz