Usually when you have errors that are 180 degrees apart, it's an indicator of backlash. The machine gets under a load and 'pulls' the lash out, then when it gets around to the opposite side it 'pushes' the lash out. My older machine does the same thing at 10 and 4 'o clock positions. I do what Steve suggest and do a couple spring passes, and it never gets under enough load to push or pull the lash. When this no longer works, or I get so sick of fighting it, it's time to do proper maintenance and fix it.