[rdorman]

Tires are my bet. The speeds in which it occurs is the norm for a dynamic balance issue. Only the best of machines can fix and diagnose the issue. Do a search, when it becomes available again, and the machine and process are described. In the mean time you can look for a missing weight. And the vibration with this issue does vary in intensity. Some times you can barely feel it, others it is pronounced. When the vibration occurs, look at your wheel, it will be shaking back and forth!
Rick