Thanks for the tip Joe.
Looking through the service records this seems to have been done. The tech at that time suggested that the next step was to change the valve assembly. We will try it again though.
During the latest work on the tranmission the clutch plates were found to be flaking. All the plates and various seals, O rings etc. have just been replaced. The Valve assembly was inspected and looked fine.
This is a 5 speed auto tranny with the part id of xxx 500 xxxx, if this helps anyone. I cannot find any record of it having been replaced. (the vehicle only has 70K and has been sitting for most of the last 5 or 6 years...probably when this problem started to develop...around 60K)
Could this be an electornic component that is failing?
'90 190E 2.6
'87 300TE (gone to car heaven)
'84 190D (passed along...still in the family and used on a daily basis)