Thanks for the welcome a10gf, and the reply.
I have heard of the "brightsparks" external condeser (I have run a couple of AMC's for a year or so, so although new to BSA'a not a complete novice with classic brit iron) and would certainly consider it if I need to replace the condenser, however if it's a windings issue will probably send the mag away for refurbishment. Either that or look at electronic ignition although I do like the mag system, not least as you can start and run with a dead battery if needed.
I'm not 100% certain it is mag yet, will investigate further including run the thing till it plays up and check the spark hot again, but it only seems to do it after 10 to 15 miles and being made to work. Another avenue I will check is compression hot and cold and take a look at the carb to see if there are issues there.
It doesnt missfire so much as lose power which gradually gets worse. No popping back or spitting, just more and more gutless untill it stops completely. Just wondered if anyone else had similar problems so I could get a few pointers.
Regarding the oil level, while I doubt a high level would cause problems like this can you give me a clue as to correct level?
Thanks
Dave