Y axis issue

Hello,
I'm new to CNC Zone so if this post belongs in a better suited forum - please let me know.

I own a Legacy Artisan CNC (maybe about 10 yrs old) with Mach3 and have been experiencing some slop in cuts I make on the machine. It is only in the Y axis that this occurs. I have checked the set screw that holds the gear on the Y- axis stepper motor shaft and it's completely tight against the flat found on the shaft. Is it possible that the stepper motor has an issue? In the attached pictures, I am cutting through 1/2" plywood with a 1/4" straight bit, taking 1/8" passes at a time. For some reason, as the bit takes deeper passes, it finally gets to what looks like the path it should be cutting. I am using V Carve Pro as my CAM software. Any thoughts are much appreciated.
Thanks!
Peter