missing after an hour

[Drew randles]

I have a jeanneau 650 leader with the cobra 4.3 V6 engine, the boat is a 93 model so I assume the age of the engine is about the same.
When I run the boat from cold she runs like a dream but after about half an hour she starts to miss fire almost as if one or two of the cylinders shut down.
Has anyone had this same problem or could suggest what it might be ?
any suggestions would be much appretiated
Thanks
Drew.

[gonzo]

How do the sparkplugs look like? Reading them will tell you a lot of how the engine is running and if the fuel/air ratio is correct.