When I lived in Toronto I bought a new Suzuki GT500 in 1977 which had galvanised metal shrouds on the spark plugs. It was a great bike but it suffered an intermittent misfire. The Suzuki Dealer had it back a couple of times to try and cure it, but he said 'all was well'. Eventually I told the Dealer that if it didn't get sorted out I'd be taking legal action to get my money back. After a couple of days, the Dealer told me that he'd tried everything but couldn't find the problem and so he left the bike running in the garage..........and as it was in a dark corner he then noticed the arcing from the plug shrouds to the cylinder head......Eureka ! The shrouds were cut off !
The bike ran like a dream after that and I rode it across Canada and the USA covering 6,000 miles in three weeks - and it never missed a beat.
No doubt you'll sort out your OEM shrouds Ash.......but at least you'll know where the fault lies if you do suffer a misfire.