If you have the service manual look up MAP sensor. It's a bugger to get to. Located on top, front of engine kind of under the dip stick. Held down by a single bolt. When replacing it is easy to think it's in properly and the bolt will tighten with the sensor slightly out of place. It is a dual sensor, IAT/MAP, that provides manifold air pressure and temperature. Costs a couple hundred dollars through Mercruiser but is a Bosch part common to various automobiles and can be had via auto parts supply for about $30. Bosch P/N 0281002845.
When I bought my boat the previous owner told me he had a problem with a MAP sensor failure and had recently had it replaced. I found that it was still wonky. When I investigated I found the previous repair had not installed it properly per above. Replaced it and have about 1200 hours on the new one with never a hickup.