Seems like there could be a few things going on with your setup, but I'm still a little unclear on the plane in which the cartridge is wobbling. You said "back and forth," but did you actually mean "side to side" as in sides of the cart moving up and down, like a row boat when you rock it? Unipivot tonearms are very susceptible to any type of vibration and are rather unstable by design; that sharp spike is the only point where the entire tonearm articulates with the base so the tonearm has nothing to provide stability to it. In ideal conditions, it is not a problem, but if your table is not isolated well, it will wobble. It's simply the nature of unipivot tonearms. I used to own a VPI Scout and now have the Classic, and the Classic, even though in a different league, is not immune to mechanical vibrations that cause the cartridge to wobble unless it's perfectly isolated. I live in an apartment building that has rather "live" floors that carry vibrations easily, and I noticed listening at night results in almost no wobbling whatsoever, but during the day the cart wobbles slightly here and there, but it does not affect the tracking ability or sound. If you don't have it already, I suggest putting the equipment rack on a slab of concrete (or 4 thick concrete tiles, about $2 each from Home Depot), then putting the Classic on a maple block and the block on 4 Isoblocks. It worked great for me.
