I had an '07 KTM 990 ADV that had the same slow frequency bounce as the O/P describes. It would go away completely at about 37 mph. It did it with 2 different Pirelli Scorpion Trail tires. I had it balanced at two different shops then tried balance beads but never got rid of the bounce and just lived with it. Finally before I sold the bike I put new Shinko 705s on it and the bounce was gone. The only thing I can think of is that I used the really heavy duty tubes in the Scorpion tires and just put a standard tube in the Shinko. I had no stiction problems on that fork.