out of balance suspension will cause this, back to stiff front to soft, changes the rake and can cause head shake. The K5 doesn't typically head shake if set up properly, are you forks pulled up in the clamps? they turn better but then you can get head shake. The CR 500 is pretty bad due to the steering head rake. I would take big belly bobs advice and set the rear sag, more than likely thats the culprit.
another trick that was used for high speed riding was to tighten the steering stem nuts a bit tighter to cause a modest amount of drag, that helped, and of course you can always get a steering damper.