Steering head bearing adjustment
Another possible cause is the steering head bearings are out of adjustment.
+1 on this idea. If they are too tight, at about 35 mph or so, the bike will go into a slow side-to-side "weave" on the road.
Too loose, and the front end will feel vague, and won't hold a line well.
REALLY too loose and you'll actually be able to check it by the method described in the Haynes manual: Put it on the center stand, and, with the front wheel off the ground, grab a fork leg and try to move it backwards and forwards to feel any "play" in the bearing.
NOTE: Simply eliminating the felt "play" as this describes does NOT have the pre-load on the bearing tight enough.
I've always used (with excellent results on over 100 motorcycles of various styles and brands) the "feel" method of setting the pre-load:
With the front wheel elevated, it should "flop" freely to either side, absent any stiff cables, handlebar mounted accessories, etc. interfering with this.
In this mode you can also check for the detent "notch" feeling as you slowly pass the handlebars through the straight ahead position.
If you DO feel much of a notch, the only fix is to replace the headset bearings AND races, as running them too loose has allowed a vertical groove to be hammered into the races.
If that's OK, experimentally tighten the preload to the point where you can actually feel some increased resistance in moving the handlebars.
Now back the pre-load off until there is no increased resistance, lock it down and ride the darned thing!
Again, to check for excessive pre-load, watch for the "weave" at about 35 mph or so.
Enjoy!
Bob
In the past few weeks I have noticed a front end wobble at decelerating speeds arouund 35 - 40 mph with my hands off bars . It has a pretty good shake to it , but I do not notice with My hands on the bars. I know I am going to get the smart ace remarks of well dont take your hands off bars , with that being said I do notice it with one hand on? trying to locate what problem maybe without having to take it into shop. I am sure that lots of us have experienced this. Any clues to help helps. 
