The 50% during cold start is normal as you are observing. The fact that after it enters closed-loop that it slowly drops to 0% indicates something is wrong.
With key to ignition, fuel pumps active, depress the throttle plate, what do you get? Then wide open throttle, what do you get?
From what I have observed on mine, it seems to have a lean stop at 18% and a rich stop at 93%. I can never achieve 0 or 100.
Since you are 1990, do you have a error code module you can check for codes?
Are you sure coolant/engine temp sensor is good? How did you test it? Did you test it both stone cold and normal operating temperature?
This is a good read also:
http://w124-zone.com/downloads/MB%20CD/W126/CIS-E%20diag.pdf