I have two 200 hp. Merc. Optis. that are 2000 year models. On the motor that I am having issues with I have recently replaced the thermostats and covers, installed a new high pressure fuel pump, installed a new low pressure electric fuel pump, cleaned the VST, replaced the fuel filter under the cowling, replaced the water seperator inside the boat, installed a new shift switch, and installed a new TPI. The issue I am having with the motor is that sometimes on a cold start the motor will fire but will rev up to a high speed on its own over 3000 RPM. While this is happening the motor sounds as if it is not firing on all cylinders and I have to turn the engine off to prevent it from revving up higher. Usually when this happens on the third or so attempt to start the motor it starts normally and idles smooth. At this point when I put the engine under a load of about 1500-2000 RPM it will run smooth for about 45 seconds to a minute and then cough, sputter, and reduce speed on its own forcing me to let it return to an idle. It will again run smooth at an idle but will not run at a higher speed for longer than a minute on every attempt. This situation seems very odd to me as I have used a DDT scanner to check the engine for any malfunctions or error codes. All of the onboard diagnostics on the DDT tell me the engine passes all tests within the requied specifications.

Any help or ides on what coud be happening here would be greatly appreciated.

Start by checking spark and compression.
Look @ spark plugs for fouling.
If both check out OK then attach fuel and air pressure gauges to rail.
If not maintaining factory spec. on either engine won't make power.
dean4now