Struggling with uneven passes on S3

Hey gang, I’m struggling with some uneven passes on my S3 during facing operations ( 1/4 flat bit, Makita router, Fusion360 CAM) . This image is of the X axis (sorry it’s rotated). The behaviour is that if the spindle is moving from the left side of the machine towards the right, it’s one height, on the return pass it’s about .3mm higher. I’ve tightened every belt, wheel and bolt. Leveled and squared the machine a few times, tried different bits, step overs, etc. The z height in the gcode doesn’t change. If I only cut from one direction (so just left to right and only travel from right to left instead of cut) I don’t get this behaviour. I’m very much a noob so I’m really pulling my hair out on what to try next. Any pointers would be very appreciated.
EDIT: My google-fu is weak evidently and it looks like someone else that had this issue suspects an out of square spindle. I’ll give that a look in the morning.
X axis uneven passes

Yeah, tram the wasteboard/spoilboard/fixture and ensure that the spindle is plumb.

Another thing to consider is the difference between climb and conventional cutting — might want to use a roughing clearance and cut to the desired dimension using a finishing pass.