Check wheel alignment using a piece of string (or a known straight edge). You could always check with a laser along the chain run if you have one. If the bearings in the steering head, swing arm and wheels are all correctly set and not worn it sounds like a wheel alignment issue or a bent frame. Did you notice any flaking paint around the headstock whilst you were rebuilding it, a giveaway of a bent/twisted frame. Did you have the forks apart and did you check for stanchion straightness. If one was bent you would have had trouble passing it through the yokes (or triple tees I think you call them across the pond).
One last maybe, are the tires new or old stock? Are they correct for the model, some bikes do not like certain makes of tire?
