First, regardless of what brand and size tires you have this has been a fairly widely reported problem on the SERG's, and most have found that the steering head bearing adjustment, when done correctly, took care of the problem. I would suggest that would be a good place to start the diagnosis procedure. If the problem persists after the head bearings are properly adjusted and the nut properly torqued (see the bulletin posted above by rascal1007), then further diagnosis will be needed.
As for the tires, what is the information molded into the sidewalls? For instance, the stock Dunlops would have 130/70 B 18 63H on the front, the rear would be 180/55 B 18 80H. There is a Metzler ME880 that matches up in the front, 130/70 B 18 63H, but there is no direct match for the rear in the catalog. The closest ones are radials, not bias ply, and they have a significantly lower load rating than the stock Dunlop. The closest bias ply listed is a 150/70 B 18. Let us know what you have.
Jerry