Like ok4550 says, a misadjusted valve can work OK for a few minutes until the engine heats up, then metal expansion causes the valve to stop seating correctly, losing compression in the cylinder and causing misfire/loss of power.
A compression test may be a pain to do on this cylinder, but if you don’t do one you won’t be able to rule out one possible cause of your problem. And you’ll need to do the compression test when the engine is hot/misfiring.