Engine problems when starting

Hello, everybody. Since March I am proud owner of an A8 4.2 Quattro, Bauj. '94. He initially ran great, then started having problems starting. He started off, then again only hard and finally no longer. A defective speed sensor was detected during reading (apparently a known problem).
My dealer, who makes a great effort, places the car at the Bosch service, which replaces the sensor. Was about a week good, then everything started again from the beginning, with the result that I went huckepack last week on the ADAC transporter from work home. A new visit to the Bosch service with 2 days stay at the engine tester incl. Diagnosis device did not produce any results. He jumped off the grid every time and there was no mistake in the memory either.
A desperate call to the Audi dealer reached an A8 specialist who obviously knows all service bulletins by heart. He immediately asked if only the sensor or the complete cable set had been exchanged. Of course, only the sensor was exchanged, cable set was reordered. Now I'm curious if it'll get better.
What I just don't quite understand is that the cable set is so important when the sensor is obviously defective...

As a gimmick, the starter is now also in the bucket, which has probably thrown the nerves through 100 times, extremely long lyres and now the free run runs the first seconds.

Maybe someone here has had similar experiences and an idea?

Greetings Peter