Sounds as if there are good and bad plugs of all makes to me. I've been running (the same) Bosch plugs in my (ironhead) A10 for over 20,000 miles and they're still better than fine, another of the same in my B31 and that's fine too, NGK B6/7/8s or Champion 5/4/3 in other things with alloy heads, and, yup, one or two very rare glitches. But that's in loadsa miles. I'm not big on changing plugs unless they've eaten their electrodes or are otherwise messed up, which usually says something else isn't how it ought to be. Sometimes I've found 'plug failure' is advance warning of a magneto (or other ignition) fault - but replacing the much-maligned two-buck item gets you home at least.
Dunno what a fuel cat is Peter, but if it's tin pellets in a canister or something I'd say it was probably snake oil, like those so-called spark boosters that wreck HT pickups and eventually destroy HT windings . . . Hope you're able to stuff a couple of noughts on the mileage to get proof out of that particular pudding!