The most common cause of these type of problems I have found is that the rear wheel and the front wheel are not in alignment. The wobble occurs at speed because the rear wheel is a heavy gyroscope and the front wheel is a lighter gyroscope. The higher the speed, the more the heavy gyroscope tries to pull the front gyroscope into alignment with itself.
Here is the solution,
http://www.ex-500.com/index.php/topic,148.0.html
I use a laser instead of string.
My 2001 is stable even when I have gone past 80 downhill. My guess is that the Power Pures probably stick to the road better than the OE Hoops so the rear gyroscope pulls harder to get the front in alignment which makes the wobbles worse.