Hello my handle bars shake up and down at 50-55mph as if the wheel is out of balance, the bars sometimes have a small amount of flap at walking pace. The bike is a 1982 xs400 maxim which I bought at Christmas, it passed its MOT a few weeks ago. The tyres and wheels look ok and the front wheel balanced on saturday. The tyres have inner tubes and set to the correct pressure. The headstock feels ok and does not judder on breaking as it would if it were loose (the MOT guy would have picked this up as he is strict). At ideas what the problem is?