Could be a slightly bowed leadscrew moving your carriage a little bit every rotation.
Can check it with a magnetic base dial indicator while it's running with a flat tip.
Try tightening the saddle gibs a bit.
Shown on page 40 of the manual.
http://cdn5.grizzly.com/manuals/g0602_m.pdf
A new leadscrew is only $52.50.
http://www.grizzly.com/products/G0602/parts/4
Hoss