Se my response to Andrew for the the difference between what I call shudder vs. your problem of high speed wobble or shimmy.

If you don't deal with, either cure or manage, your high speed shimmy it will cause handling problems and on long descents might get bad enough to cause a crash if you don't slow down out of it.

Try the various suggestions, such as making sure your headset is tight, lowering front tire pressure slightly (if going to 85-90psi doesn't help, don't bother going lower) shifting more weight forward, or to the BB, and/or bracing one knee against the top tube. Also check wheel alignment & play in hub bearings, especially front. Also spin your wheels while looking end on at your tires. If there's any wobble in the tire o tread either because of poor construction or poor seating, that will move the contact patch side to side as it rolls setting up the vibrations.

None of this involves spending money or replacing anything, so take your time and experiment. See what helps and what doesn't and see if you can cure the problem or at least get it manageable. Then if it's still a problem, come back and post what you did that helped, and that might point a direction for further investigation.