Get the vehicle to miss whilst parked in the driveway. Pull the plug leads off one at a time while it's missing - if the miss gets worse, then that plug lead is not the culprit. if the miss gets no worse when you pull a lead, you've found the crook lead/plug.
Is the rotor button sitting down properly? New Bosch rotor buttons are pretty snug, and if "sitting up" can cause an intermittant miss. Slop in the dizzy main shaft can do the same (don't ask me how I know
).
Has someone stuffed around with the carburettor at all? If it's lunning lean, it may surge or miss (particularly if someone has farted around with a variable main metering jet). If it's got a variable main, you can screw it out a bit and see if the miss goes away.
Tried hunting down a miss on a 308 VK Calais at one stage... spent a small fortune replacing electricals... still couldn't find it. Ended up being a worn cam lobe leaving the exhaust valve shut (Ace's compression test should help pick that one).
Cheers,
Harv