Before you take it out, have you tried cleaning the slip ring?
I had it happen on the freshly rebuilt CSR recently, the brushes in the (repro
) pickups were so soft that it would only take about 5 miles until she started to misfire, then finally stopped after another 2 or 3 miles. I took out one pickup, cleaned the slip ring by inserting a piece of cloth soaked in petrol, turning the engine over (cut-out button pressed!!!), and off we went again, for another 8-10 miles until the whole thing started all over again.
I replaced the brushes with a set from SRM and presto, no more problems ever since. If you ever happen to find pickup brushes that will draw thick black lines like a soft artist's pencil, throw them away! The good ones will leave almost no perceptible line at all when rubbed over a piece of paper.
It is just a small chance, more likely the mag is faulty indeed, but who knows, worth a try at least. I reckon you have checked everything else (cut-out contact, earthing brush, plugs, HT leads, etc, etc)?
Cheers, Markus
