Well, actually W140s will work with you if you do read all codes carefully. There are two typical readings of codes: a short 15 minute express check up and a comprehensive almost 2-hour system diagnosis.

Most independents don't like to do the long test (to time consuming), but according to your symptoms, I would take the car to the dealer and have the full test done before changing every single accessory on your car.

If there are NO significant codes, then I would begin to worry, since the task becomes real difficult. It could be a short somewhere, a reversed connector or something really small that codes will not detect but will make your car misbehave such as bad as you describe.

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A. Rosich
CL 500, 1998
S 500 L, 1998
E 320 T, 1995 [Sadly sold ]