Originally Posted by gtbensley
It spins fine and when the front is suspended it doesn't seem to have slop.
There have been some great answers on your headshake problem. Google .headshake' and you'll find many different make and model bikes do this around 40-50mph, especially while decellerating.
I have a Honda ST1100 and the 50K+ miles I rode it, I have run 3 different front tires. The OEM tire had no headshake, Bridgestone BT023s have NO headshake, while Avon Storm and Storm ultras shake like a dog crapping a peach seed. I know of dozens of ST1100s that can not run Avon Storms on the front, doesn't maytter if the head bearings are stock or changed to tappered roller bearings.
You could have mechanical problems, but i'll bet a dollar to a doughnut that the bike doesn't like that front tire.