Could be a lot of things. You need to do a thorough diagnosis, with the assistance of the Factory service manual, Part # 003N210530 $26.52, available from any dealer, including me. You did not mention specific model, but the MFS30A is the carbed 30 hp 4-stroke motor. The ECU on the A model 30's is basically a glorified CD module. It can limit RPM due to error/problem conditions, such as temperature and oil pressure, but it is not in charge of fuel management -- the carbs do that mechanically. The ECU does control the advance curve, so a check with a timing light would tell you a lot. Do you run the carbs dry at the end of each day? Do you drain the carbs for the off season? If not, a likely culprit is varnishing in one or more of the carbs. Do you have an overheat buzzer sounding (if EP type)? The ECU will limit RPM to about 2800 if overheat is detected. Will the motor go over 3,000 RPM without choke? If not, you are probably starving for fuel. Are you running fresh fuel, and do you have a water-separating filter (with fresh cartridge) installed? A slight water contamination in the fuel might contribute to your symptom. If the motor oil is overfilled, that can also produce a symptom such as yours, as a result of high crankcase pressures and sloshing of oil in the crankcase.