I think I may have mentioned before I had a similar failure with my RS250- no load spark test produced a fat loud spark and it looked like everything was happy, but the bike ran poorly if at all. The affected cylinder would have a wet plug and it looked a ll the world like a fueling issue. I end up replacing the coils as the connector to them was a fussy proprietary HRC bit and was a bit damaged- the solution was to replace the coils and the entire harness. It would seem more likely that a coil would produce intermittent results than a CDI box- in my experience the CDI components are almost binary in use, either they work or they don't. If you haven't already swapped out the coil it might be an easy and cheap(ish) thing to try. At this point we are all amazed you have had the nerve to stick with this as long as you have- and I think we all want to encourage you to not throw in the towel. Also we all want to know the answer.