White line thru center of mesh plots

[ferrellsl]

I'm getting these white lines thru the center of my plots. It seems to happen on large mesh files. By larger, I mean greater than 2000x2000. It also happens on smaller meshes when the resolution is lowered, as in the 3rd and 4th images which is a 1200x1200 mesh.

EDIT: apparently the white lines appear on any mesh whenever a lower than full-resolution is selected.