Dear Steve and all,
Thanks again for all your help on this posting.
With the help on my tech, we did some work on the front end Friday in an effort to get it to stay on the road.
He first replaced the very badly worn steering damper, which did not have any effect. We then replaced the steering fluid and filter. The fluid was a little low and old, and the filter was definitely past due for changing. When my tech said this would help - I was skeptical to say the least. To my surprise it made a significant difference to the overall feel and has reduced the overall tendancy for the car to float all over the road. It did not fix it perfectly but I would say it is 60-70% better.
One of the ball joints did have a cracked boot so I assume we will tackle this and the idler arm bushings one day soon it I keep the car for a while longer. (always on the lookout for the right W140 S500!)