M.B.DOC, thanks much for the tip ... I think you got a point with the sensors - did some more extensive testing yesterday after it got dark outside and the remote locking system worked flawlessly without a single miss - during daylight completely intermittent.

I know that MB doesn't offer the sensors separately - you only can get them together with the complete door handle (= very expensive).

I will keep my eyes open for a set of good used ones.

Now, do you happen to know if the sensors have stayed the same throughout the whole R129 model life cycle or have there been any changes between the early and late cars?
Also, I'm assuming that MB did use the same sensor assemblies in other car models (cost optimization) as well, do you know which ones?
Someone told me W124 and W140 up to 1996 - can you confirm this?

Muchas Gracias.