Tom,
When my 1990 300 SL had the same symptoms, the dealer diagnosed the problem as the SRS Control Module - For Airbag & Seatbelt. The module was replaced just about one year ago and I haven't had the problem again. Tell your friend to try everything else first. The module cost $1281.00 and the labor is about 1.5 hours.

Bob

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BobH 90 R129