By the way is the distance between the chain and the frame constant? Or does it come in close then go away from the frame's tubing? If the former then your wheel just needs an alignment. If the latter then it may be the crank wheel. It may be bent.

BTW it's quite normal for the wheel to come out of alignment on bikes such as this, with horizontal drop outs. This happens if the lock nuts are too loose for the force you're putting out. I believe I had the same issue when I took delivery of one of my Dahons.

Also, since you need to loosen the cable and recalibrate the hub when you change flats (sometimes), might as well try it right off the bat.