Slight Banding when engraving

Hey Gang!

I am getting slight banding as seen in the photo (zoomed in you really cannot see it, but zoomed out you can). It only happens (or is noticeable) on images that have a solid gradient or solid grey tone. Anything else and its perfect. The banding is happening in the X axis.

Belts seem nice and taught, and I am at a loss. Motor perhaps? Any input would be greatly appreciated. My machine is a Blue and White 50W (with an upgraded PSU and 60w tube).

Thank you!!!