The general consensus is that platium plugs aren't for older Mercedes (or even some newer ones) What I've heard is that the tiny tips require a higher firing voltage and are more easily fouled. That said, I've been running Bosch platinums in my 1972 250 / 2.8-6cyl for several years. However, my 250, unlike your SL, has an electronic ignition with a, supposedly, hotter coil. If you changed back to the standard plugs and it runs OK, I'd stick with them.

Happy Motoring, Mark

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DrDKW