I've worked on a few of the older versions of this machine, which are pretty much identical in design and it will most likely be the Homing sensor (proximity) as failed and the juddering is the steppers stalling when reachs ends. The steppers are weak and it doesn't take much.

Also if it's the long axis then could be a lack of grease and crap in the rails and ball-nut, mostly because of they are bastards to work on. The steppers are only just strong enough and if the screw or bearings bind just a little they stall.

The other thing it could be if he's calling Y-axis the one that goes across Gantry is the timing belt. They connect the motor to the ball screw with a short timing belt and they do wear.

They are horrible to work on.!