An engine that is warmed up will be "harder" to fire than when it is cold. Has anything been done with the spark plugs/ coils yet?
FYI: I never like to guess, however failing coils are a common problem and that is the first thing that comes to my mind when reading your description.
Good luck! If I can do anything else for you, please let me know.
If there was no problem prior to changing the plugs, then the first thing I would do is swap the plug from the misfiring cylinder with another to confirm that the plug is not the issue.
The temp of the engine will not affect the performance of the coil. Rather, an under performing coil could be masked by a cooler, easier to fire cylinder. I would suspect the coil, if the spark plug tests passes.