Fixed it!

It was nothing more complicated than a rotor arm that had started breaking down under load, fitted a new one and she's running as smooth as ever.

Thanks for everyone's help (on this and other folk's posts) - would have struggled without you all!

Cheers, Martin

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1990 420SEL 217k Miles
1975 Chevy K10 400ci SBC power!
1995 Jeep Cherokee 4.0 Limited
1997 Suzuki GSXR750 SRAD
1989 Kawasaki GT550G4