Managed to do a little more checking this afternoon. As I know I heard misfires out the left side (might also be on the right side, but the left is closer to my ears), I decided to work on that side first.
Pulled all plugs from left side. One is shown below in the picture (sorry it's a little blurry). They all look the same.
Cap looks fine as did the rotor button and the general internals of the distributor... oh darn... must go look again and make sure I didn't lose an advance spring.
Removed left hand side valve cover and looked for broken springs, anything loose, etc. Everything looked fine.
Looked at all the wires. The boot ends had a fair bit of oil contamination and felt sticky. I assume this was from hot oil with a bit of sticky gasket sealer dripping down from the valve cover gasket.
So... only thing at all suspicious was the wires. It's looking like as I can't find anything definite, I might as well install some new wires.
Odd though... how all works well until 4500 rpm........