Chris,
Might be a long shot for you. Hopefully you're a bit more thorough than I.
Check your dizzy cap.
I replaced mine for a new one about a year or so ago (only done about 3000 miles on it) Long story short, stripped my bay out to change throttle sensor. Had a clearer view of cap, noticed full on crack across top. Misfire solved. Funny really, but the noise it was making when it did it was very harsh and buzzy, I don't know like a factory bar coder or something like AN ELECTRICAL DISHARGE of some kind. What an idiot. Still, I had a lumpy idle misfire late last year along with the under load misfire, and that was down to the loom and the bulkhead plugs, therefore making me feel better for the ball ache that was building a loom.