Welcome to the forums, nice to see another one resurrected.
As to riding in the rain I had the same problem on my CB750K1, I tried changing everything and then one night I looked under the tank and it was like a lightning storm under there loads of arcing. So I sealed the HT leads where they entered the coil and problem solved.
Before this I tired changing the plug caps coating the leads with WD40, I even protected them with sections of plastic hose pipe nothing worked, most embarrassing going over the M62 on one cylinder.
Cheers
Dennis