if a plug is dodgy it is likely to go down completely, yeah that would do it as far as 25mm top speed goes but it would also 'not sound right' or feel right for that matter!
Its an easy one to test , when the bike is running dab the exhaust header with a damp rag, the running cylinder will have a hot exhaust and you will hear the hiss of the damp rag touching it, obviously a cylinder not firing will have a cool exhaust so this wont hiss. Its a good method of diagnosing cylinders that are not firing on multi-cylinder bikes. dont be dabbing any heat shields if they are fitted, you need to get to the pipe as it exits the cylinder if poss