I will see if I can decipher some of the Workshop Manual information this evening to help you out.
I don't think you have done anything wrong at all. Certainly, tightening the connections previously won't have harmed the contacts, only made them harder to pull apart!
Pitting and blackening of the contacts would normally suggest loose fittings/arcing though, at that could cause a resistance to build up. Have you checked the regulator/rectifier, and the output from the charging system. If the voltage is too high (should be around 14.2v - no spikes when revved) or too low, this can cause major problems.
If the worst comes to the worst, I'll at least e-mail you scans from the Manual.
It's curious that swapping out the coil fixed the problem, then swapping back showed no difference. This would lead me to assume that it's not a necessarily a coil fault; maybe a pick-up coil or the stator instead. I had a stator failure once, but it stuttered on load and at high RPM. There is something in the manual about all this though... I'll get on it tonight.
If you need a new harness, I think I have one. In the mean time, see if you can try John's PGM!
Andy.
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