The cooler cylinder is firing, but not as well as the other cylinders. You may not feel it, there may not be a miss, but there is a power loss.
This could be a partial misfire due to a spark plug, wire, slight vacuum leak, all the usual suspects. It could also be weak compression in that cylinder. I would do an ignition inspection, and while you have the plugs out do a compression test on the entire engine. Don't do just one cylinder, you want a good comparison between cylinders. Let us know what you find and we go from there.