2003 DF15 - one cylinder on again, off again

[Irish Rogue]

My DF15 has started exhibiting a new (and unfortunate) behavior.... Instead of running along with both cylinders firing, it will drop back to a single cylinder... Additional info:

* More pronounced when cold, especially at first startup, but does not disappear entirely when warm.

* When the engine drops back to a single cylinder, it won't be a single miss, it will start missing on that 2nd cylinder for 2-30 seconds consistently, then return to normal operation

* The cylinder that's missing is the LOWER of the two in the engine. I know this because I pulled the sparkplug wires off when it was exhibiting the behavior... Pull the top wire and the engine died immediately. Pull the lower wire and the engine carried on.

* Plugs look clean/okay (though I've not changed them), I've put a spark detector inline on the lower wire and it shows good, I also got a nice little jolt when I pulled off that lower wire with the engine running (d'oh!)

I have the service manual, and am ready to dive in and tackle this -- but am looking for some more expert opinions on where I might go first?

[bezerk]

change the plugs .

[boats.net]

Start with changing the spark plugs.

[Irish Rogue]

Well you guys get **** stars. It really WAS that simple. Pair of new plugs and she's running beautifully. I was ready to do some major diagnostic work!

Many MANY thanks guys, for the expertise.