I had a old z1000 a few years back that suffered these intermittent problems, and the fix given to me by a old zed head was brilliantly simple. As the fellers have said the low tension side of things doesn't take a lot to drastically alter the high tension side. If your getting say 11 volts at the coils instead of 12, multiplying many many times over and being converted to high tension that one volt loss becomes a helluva lot more. If you check the voltage to the coils ( low tension) you may well find there's a drop, caused by old wiring and old joints leaking off voltage which is what my problem was. Fix was to run a live feed to the coils direct from the battery via a relay, useing the original coil live feed as the switch wire. Sounds simple because it is, but it worked, plugs ran a lot cleaner too. Just another thing to look into mate :-) also, that bike ran dreadfully on iridium plugs, as have a few I've owned, no idea why, some bikes just don't like the buggers..:-)