I also have the same problem when riding on the highways, it come and go's. It seems to be on windy days or when passing or being pass by trucks. I read somewhere it's the short wheelbase and the front fender. I loosen up on the bars and try to ride through it. It's never got so bad that I lost control. It seems to be the nature of the bike.

 

Quite a while ago I had a lazy handlebar wobble under 30 mph, the bars just wanted to wander left and right a little bit (maybe an inch at the bar end) and my tires were at 15 psi. I've got them at the recommended 22 now and have no problems from 0-70. I don't ride faster than 70 as thats past 3/4 throttle in 6th to get there and I don't like to hold it open for an extended period so with stock gearing I limit myself to 70-75 max but generally 60-65 and its a smooth ride and after 1200 miles the wind doesn't bother me anymore.