This may not apply to your 88 124 but what I finally found on my 84 500SEL with the same problem was the steering box was not centered when the car was going straight. Somewhere in its life the relationship between the steering wheel, steering shaft, and the steering box got out of alignment with each other. Most alignment shops will lock the steering wheel straight and alignment to it.
An easy way to find out is install an alignment tool in the steering box and see if the steering wheel is straight.

Look at step 42 in the following DIY for resealing a Mercedes steering box. It shows the alignment tool (just a pointed bolt) and where it is inserted.

How to reseal a MB steering gear

If the steering wheel is not straight with the alignment bolt installed, you have probably found your problem.

PaulM

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84 500 SEL (307,xxx miles)