It could be something as simple as a bad ground/earth. Clean and check all the connections first.
What voltage does the battery show after a ride?
There is a possibility that the stator is failing. I had a stator burn out and the performance degraded very slowly over a number of months, but the rest of the system checked out fine! It wasn't until a couple of the windings fully burnt out and the damage became visible that I was able to track it down. That was some 15 years ago too though, and there wasn't any (English) NSR info back then, and my NSR spannering skills were in even more question than they are now!
I expect it's more likely a bad/corroded earth.
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Andy.
NSR-WORLD.COM
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