High speed front end shimmy is a resonance problem. As Pierce pointed out there are any number of input sources, including tires, minor wheel alignment issues, overall frame alignment. The cure is to address the input sources as best you can can and/or change the natural harmonic frequency of your bike. An old school trick is to put a knee against the top tube as a damper when coasting at high speed. Other things that can help is transferring some weight to the pedals, moving your body forward a bit to increase front wheel drag.

With a bit of experimenting you should be able to get a handle on managing the problem, though it may never go away completely. My old bike shimmied at about 35mph, but stopped above that, so I made it a point to either slow down or speed up a bit to stay away from the specific problem speed.