Can we get a screen-grab of your settings from your last job, and a screen-grab of the geometry in MeshCAM?
Also, there have been mis-labeled cutters, and as I’ve pointed out elsewhere, there’s the matter of tolerances to keep in mind and also material deflection—if you’re cutting at 100% tool-width, which would be .125" or .063", depending on if you’re using the 1/8 or 1/16 tools, you could end up with very thin walls of material if the cutter is under-size and out of spec. and there is deflection in the material. That shouldn’t happen, but it could. Also, if the ridges are always aligned with the X axis, you may have a bit of play in your Y-axis carriage belt, so I’d say wiggle the table with the machine on, and see if it will move. If it’s snug and won’t slide at all, then we can rule that out. If it moves, then you may need to shim the belt in the belt-clamp to take the play out, which we can walk you through.
You want your step-overs to be 50-80% for most things, which will leave some cusps if you’re roughing with a ball-end tool, but it shouldn’t leave free-standing walls like that.
Looking forward to some screenshots to sleuth to the bottom of this