Dear Declan,

There are two answers in here that I like. It is either a coil or you are
getting water in the cylinder do to a leaky head gasket. It is a cylinder at
the top of the bank that would have gotten the hottest if the motor was ever
heated. So you could have a warpage problem. Most likely not that
complicated.

Try replacing your spark plugs after you run the motor for a few minutes. Look
for water on the plugs. If you have no water on the plugs start going through
each of the coils and cleaning the ground connections and retrying them. Make
sure they are good clean connections. That could be the only problem.

If that works great. If it doesn't next step would be you have to figure out
which cylinder is doing it. So with your engine cover off and the motor at an
idle and run long enough that the problem is showing pull the boots off one
plug at a time( use plastic plyers or good rubber handle plyers). Obviously
the engine should drop off and sound different with each plug you pull off
reconnect them if it does. If the motor doesn't drop off you have your bad
cylinder.

I would pull the coil and to check your work either swap it with another coil
on the motor or take it to a outboard shop and have them do a coil test.

Good luck