Mine has done that for 5 years, with two sets of tires. I know my Avon Venoms are good tires. I disassembled and greased and adjusted the head bearings. I have installed Progressive fork springs and 440 shocks (but not for that reason). No change on the handlebar shake at about 40 mph. It only needs one hand on the handlebar to control it, so it is not that bad, just unusal/spooky. So, like someone said, don't let go of the handlebars.
