HDPI Fouled plugs help!!

2006...replaced a coil and plug but after 2 runs am does 600 rpms less...
can anyone list (maybe in most to least likely)...causes for this...3 to 5 at least, more is better!
Thanks!

[boats.net]

Hard to say, Check compression, spark test, check fuel system(vacumn and pressure test and condition of fuel), oil injection system.