1997 200 efi merc.no power under load

[laguna21dc]

I just took out my boat after changing the switch boxes.and trigger,checking spark with a timing light and spark checker and it runs ok(it seems) on the trailer ,but has no power and won't rev up past 3500 rpm under load?What could I be missing?I sqeezed the bulb for fuel and it made no change.It feels like a misfire problem.Is there something in the alarm syestem that could cause the engine to run on half of the cylinders under load when it is in gear, but not on the trailer?How can I check it under load without taking it out to the lake?.....ready to dump it in the ocean!!!