Well I took my CX500 for a ride yesterday, and once it got near the 6K RPM mark (around 65 MPH in top gear) under load, it missed and farted and burped, so I turned around and came home again. I pulled the carbs and cleaned them for the second time in a week, even though they were spotlessly clean when I pulled them apart. Anyhoo, I put them back on, took it for a ride, and, it had made absolutely no difference.
This morning, while ratting through a box of old CB750 ignition parts I found a pair of coils with decent looking plug caps on them. Not CX500, but they looked (just) long enough. I pulled them apart and they still had the original resistors in them, and they looked nice and clean. I swapped them out and took my bike for a ride around the block, and voila! Problem (apparently) solved. The engine now revs stright thru to redline, (9750 RPM) with no hesitation and stalling at the 6K mark. Excellent. I think I better go for my first decent ride since I bought it. Cheers, Terry.