Interesting Effect on large object sides

So I've noticed an interesting pattern that only shows up when I print long items like boxes for things or large flat brackets. I'm seeing a sort of wood grain effect across the long sides. It doesn't show up or affect any of the smaller objects, only on these large ones on the long sides. It looks rather interesting, but I'd still like to figure out what is causing these lines. The pictures are of the PanelDue 7" enclosure that I printed last night. The STL files that printed were generated from Onshape so I know the grain isn't in the STL file, and what is bizzare is how it moves up the Z axis and across the XY axis over the height of the object. The lines are not perpendicular to the bed either.

The horizontal line look like Z wobble or the effect of poorly regulated bed temperature. The diagonal lines are typical of extruder motor current set too high or too low.

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Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]