X2 on the alignment. If the toe-in is set too high (more narrow at the front than the rear) then the car will tend to walk around on the road. You may also see some feathering on the outside tread. No idea on what you have for tires, but really wide tires will cause this as well, as the tires will try to settle in the road wear grooves.
A better solution to the loosening nuts than tack welding would be to drill a small hole through the ball joint bolt, then use a castle nut and a cotter pin to keep it from spinning loose.
And, were they the actual nuts from the dealer, or were they something like the Help! nuts they sell at the parts store? If they weren't the ones from the dealer, then the nuts may have been made with substandard steel and the threads may not be strong enough to hold.
Hope this helps.