I do my own work for the reasons most people on this forum do their own work. However, there are times when inexperience and/or lack of proper equipment will likely cost much more time and money than it's worth to 'chase the ghost'. Your problem is one of those that can be very difficult to solve - as you're finding out.

When I had a similar problem with my 90 SEL, I made an appointment with a local Bosch trained shop that I trust. They diagnosed the problem in 30 minutes and I did the repair myself. If I remember correctly, the diagnosis was $60.00.

Oh yeah - problem was that a mouse had chewed a tiny bit out of one of the computer links (cable) that runs on top of the left inner fender. It could BARELY be seen, and only because the diagnostic computer definitely prooved that the correct signals were not getting where they were supposed to be. With that info, it drastically reduced the troubleshooting and 'sport replacement' of various components.

As much as I enjoy keeping things running properly and doing the work myself, sometimes you've got to weigh the risks and decide if it's worth the time and effort and costs of potential mistakes.

Good luck - and let us know what happens.