The tires are new Firestone Deluxe Champion made from polyester I think but definitely not nylon. Being bias-ply I was expecting the cold ride bump but they don't even do that. They are smooth.
This problem occurs at all speeds and again I want to stress that it is not a severe ride, merely a soft oscillation. I am thinking as you, maybe bent axle. It is a torque tube and the bushings at the front end are new.
