Bike shaking while decending

Is the stem, spacers, headset and frame set tightly on the fork?  I had a problem where everything was loose (turned out to be a problem with the headset) and this caused the bike to be really unstable on descents.

Clamping your knees will help with the control of the bike.

What you are experiencing is harmonics.  Right before the bike reaches a multiple of it's harmonics, it will shake like the dickins.  This is counter intuitive, but it works.  Actually release your grip on the bike.  The tighter you grip, the worse it will be.  Just relax your hands and the bike will settle into it's Harmonic.  My TP had this happen at LP on the descent and it's scary.

So relax and let it pass.  Easy to say, hard to do.

I had that happen once last year steep hill at about 45+ mph I checked my wheels and I think it was due to the bearing in the front wheel being a little loose. I don't know about all wheels but I have Mavic Ksyrium's and they have an adjustable bearing. I adjusted them and that seemed to make a big difference.... I may have snugged a few spokes up a little as well but I'm not positive on that now.

Good luck!

Speed wobbles suck! http://en.wikipedia.org/wiki/Speed_wobble

They're bad on a bicycle, but my first experience with them was when I first started riding a motorcycle in my early 20's - first time ever, hit about 120km/h and the front end wanted to shake itself loose!

do check all the mechanical first.  I do suspect harmoncis though.  Especially since it had a certain speed it got bad at and it lessened on either side of it.  I've had my road bike above 50 plenty of times.  A few months ago, I had it out going down a 2 mile 10% hill.  It was the first time I had hit those speeds with the clip ons attached.  Apparently, they altered the resonant frequency of the front end just enough to make 50 mph the sweet spot.  As I was speeding up, I was in the bars but decided I'd rather have brake control, so I went to the drops instead.  Two seconds later she started to VIOLENTLY oscillate.  We're talking front wheel covering two or three feet side to side.  Damn near lost it.  I clamped my knees on the top tube and loosened the grip on the bars while hitting the brakes.

When I got home, I took the bars off and she calmed back down.  No more problems.

Since harmonics are affected by even small changes in weight distribution and resonant frequencies of individual parts, you could probably change the harmonic response by something as simple as a different style of bull horns, or weighted bar end plugs.