Small Engines (Lawn Mowers, etc.): ferris with kawasaki 25 hp stalling, sea foam additive, spark test

Question
my ferris which has a kawasaki 25 hp will run great sometimes, usually when it is cold, however after about an hour, to an hour and a half, it sounds like the engine is cutting down to half throttle, on flat ground it does pretty good but does not have full power, but when making turns and going up hills or heavy grass it almost stalls out completely.  i have replaced plugs, and now one of them looks black and wet on the right while the one on the left is still clean, i have replaced the air filter and it now has what appears to be oil? gas on the bottom of it.  I have replaced the fuel filter and now replaced the fuel pump.  Changed out new gas and added sea foam additive.  Any ideas? I dont think it is the motor because sometimes it runs like new.

Answer
I think that the problem is that the one cylinder with the clean plug is not firing.  The first thing I would do is a compression test on each cylinder.  If that is good then move onto an ignition/spark test on each plug wire.  It may be best to try the "spark" test when the engine starts acting up.  We need to find out why the one plug is still "clean"