Insulation does not last forever, and if you have no evidence that the wires have ever been changed, at 15+ years of age, they are highly suspect. If you have a record that they have been changed within the last 10 years, verify that they are OEM replacements (dealer or Beru), and then move on to something else. If original, arcing is an obvious sign, but they are past their useful life prior to that point.

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1988 300E
1993 300SL
1994 540i

If I have a problem with my Mercedes, I go to MercedesShop. If I have a problem with my BMW, I go to BimmerNut.
If I have a probem with my Dell, I reboot it. I was an engineer for them for 10 years and we blamed everything on Microsoft.