FWIW, I rebuilt both of my S1s over the weekend. Lots of things needed fixing.
Probably the most odd was the binding/wobble in the left rear wheel. Traced it back to the "Sport" drive shaft on that corner. The shaft was not able to rotate straight as the shaft would never actually straighten out. I found that it had a set screw backing out of the center pin in the connection.
Here's the thing, I checked my other drive shafts and they didn't have this set screw. I checked the manual (it shows you have to build them, which we don't) and it doesn't show any set screw. For some reason, someone in China decided to put a screw in this one and it was backing out.