does the rev counter stay steady when it's missing, or does the reading get erratic?
If the Rev counter stays steady then it's a fuelling problem or HT problem (dist cap, leads, plugs). However, HT problem is unlikely if it behaves OK under load..
If the rev counter is all jumpy when it's misfiring, then it's likely to be a non- HT electrical issue - crank or phase sensor, poor earth connection on the ignition or ECU parts of the loom, ignition amplifier etc...