Demons in the wheel

[Chris Newbold <cnewbold@eznet.net>]

Well, my '93 90CS has finally come down with some symptoms
worthy of an Audi (read truely bizzaar)... :-(

The problem is shaking in the steering wheel and front end,
sometimes violent. It only happens when the ambient temp.
is in the mid-50s and the car is cold (i.e. left overnight
or all day). The shaking only manifests itself if I drive
at a steady speed betweem 55-60MPH. After 30 sec. or so at
such a speed, the steering wheel and front end begin to shake
and ocilate violently. The symptoms pass after a minute or
so, or if I slow down or speed up. Once the car is warm, it
exhibits no symptoms. 

If the ambient temp. is below the low-50s, the car is
asymptomatic. Warmer than the high-50s, and the shaking cuts in
at a higher road speed. Around 60 degrees, I have to get up
to 63-65MPH before it occurs, and it is far less violent. Any
warmer and the shaking is minor, like unbalanced tires and
doesn't ever go away.

The problem has persisted accross two sets of tires and three
wheel-balancing attempts. I doubt this is just a wheel balance
issue, though, because of the way it comes and goes.

I thought maybe a sticky brake caliper, but I can cruise at
80MPH in 90+ degree weather for over an hour without trouble.
So, I don't think that's too likely.

The '93 90s have more conventional hydraulics (no bombs or
30 bazilion PSI pumps and such), but I'm beginning to wonder.
Could this be something wrong with the power steering?
I don't know for sure if the 90s use servotronic, but I don't
belive so.

Has anyone *ever* seem anything like this? Any ideas where to
start looking? HELP!

TIA,
-Chris