I have the same problem with my Performax, haven't solved the problem yet. It doesn't have springs, just a tightener on each side of the out feed roller to adjust tension on one side or the other. I haven't found the "sweet spot" that keeps it in the middle yet.I was able to grab a Ryobi WDS1600 drum sander for a VERY reasonable price. Everything works well on it, but it has an annoying tracking problem on the feed belt. This is the reason the previous owner wanted it gone.
I have found a couple things wrong, one of the roller brackets was bent, and the adjusting springs were missing. I am experimenting with some aftermarket springs, trying to dial in on a good amount of tension.
I have most of the tracking resolved, but it will still walk from one side to the other after a while.
Has anybody else played with the Ryobi or Steel City drum sanders and had a similar issue that they have resolved?