I'd try to look at the bike from the rear and see if you can tell if the rear wheel is aligned vertically with the seat tube. Also, try rotating the axle to different positions and re-setting it. It doesn't take much to throw the wheel off center at the brake bridge. If there is a problem there, and you're sure it's with the frame, some people grind a little material from the dropout. I filed the threads off the "top" of one side of the axle once.