Andy, the parts that are most renewed when there's actually no need to, are coils. At least that is the impression I have. For some reason people tend to suspect them rightaway. I think it's because they do their work invisibly with such mysteries like magnetism. Don't get me wrong: coils can fail, but if I were you, I'd look at some other things first. How old are the sparkplugs? What do the sparkplug noses look like? What type plugs do you run. The 'R' types are not always reliable. How old are the plugcaps? Are the caps well connected to the HT wires and what resistance do they show? Five to seven kΩ is OK. Over 8 kΩ is no good.
I don't know the Dyna ignition very well, but if there's a condenser in it, know that condensers are more likely to fail than coils. You can always swap the 1+4 coil with the 2+3 one and see if the problem follows. CB500/550 owners must also be aware of possible shorting to ground either by the plugcaps or their wires. But that's the easiest to detect. In the dark you will see arcing and you'll hear the engine miss.
