You seem to be having more problems and questions with your 496, than anyone else on the forum!
You need to stop listening and taking bad advise from people in the industry and on boating forums who don't know enough about Merc 496's and how to find and fix problems than they know how to build rocketships for space travel!
Stock unmodified Merc PCM 555's that run Merc 496's don't ever generally have issues that are causing your condtions and problems. Most likely buying and changing you expensive ECM won't do a damn thing for your problem.
Get a scan tool or get to a good shop with a scan tool and download any codes or issues that the ecm has recorded or shows with your engine. If their are none and the engine is not in the Guardian mode or at reduced power levels with a beeping alarm if hooked up? Then your problem is not the ecm, but most likely somewhere else such as fuel delivery and injection system, filters, clogs, bad fuel, injectors, etc., or in your throttle or possibly something as routine as bad sparkplugs and ignition misses. In any event , start with the known simple things and don't start just replacing expensive parts and pieces until you have isolated the real problem! If these checks are beyond your knowledge or training for both you or your chosen shop, then get to a trained, knowlegable Merc 496 technician in your area and let him do his job for you.
Best Of Luck!
Don't attempt Surgery unless your a knowledgable surgeon!
Ray @ Raylar