How does the tire wear? Scalloped tread pattern? wearing on one edge of tread blocks?
Sounds like forks aren't level. Check them for length, against each other. Check length each one pokes out above and below triple tree. Make sure all bolts are nipped up tight.
Get wheel balanced, by a reputable bike shop.
I have been able to ride all my CX's at 55 mph, throttle lock, no hands. Steady as a rock, even at slower speeds.
Anyone else???