Intermittant vibration in Steering

I have a 1988 635 CS Automatic, that is having a intermittant vibration in the steering. I am looking for possible causes, and history.
The car just had a thousand dollars of suspension work done and the vibration is still there.
I have had the wheels balanced by not only the shop that did the work, but by another shop, that I fully trust. There is no uneven wear on the tires.
The car drives fine on city roads, get on the freeway and about ten miles down the road the car will get a vibration in the steering. I accelerate through the vibration, and it goes away. I try to duplicate it going up and down the speed range 45 to 85 mph, and cannot duplicate it.
My thoughts are that it is not tire or suspension related but having something to do with the engine, that causes the power steering to operate funny causing the vibration.
What are your thoughts?

engine/gearbox mounts?propshaft u/js,centre bearing?

Well the e24 is know to have two issues.

Lower front steering arm bushings. OE was rubber and deteriorates.

Most get Urethane bushing and fix the issue.
Here is Grunts Bushings.

http://www.gruntsbushings.com/index.html

If the wheel has been vibrating a long time you will also have other issues.

The steering gear has a loop mount that will fatigue and break with vibration. The cure is a kit I sell.

Here is a link.

http://www.roadfly.org/bmw/classifieds/general/detview.php?view=11616

Here are a couple of reviews from previous installers.

http://forums.roadfly.org/bmw/forums/e28/forum.php?postid=874342&page=1

http://bimmer.roadfly.org/bmw/forums/e24/forum.php?postid=1759995&page=1

Later,