I have had two experiences with the auto-leveling system; one in an '80 300TD wagon, and one in my '93 500SEL. The wagon rode hard and the 140 seemed to "sag" at first, then went "bouncy" similar to Hamish's situation. In both cases the accumulators were replaced and the problem was solved. I have been told by techs that the struts very rarely go bad, and there are not many other components in the system as I recall.
I would change the accumulators, but I wonder why the three techs didn't tell you the same thing if they told you the struts were OK? Did the change in ride occur before of after you had the fluid changed? Just curious. BTW, I did the job on the wagon and it was not hard to do. Probably should get the procedure from here or a shop manual to be sure of the steps if you decide to do this yourself IMHO.