compedgemarine

trying to help a friend out with 496's which are not my area. one of them started dropping rpm at wot. about 300 to 400 for a few seconds and then it would come back. now the point where it happens is down to 3500 and up. he said that it will run up and then the one will drop rpm. when it comes back it is like a switch is thrown and the rpm comes up instantly. he has a rinda scanner and it does not show any codes. any one have an idea of where to start or if there is a known issues to look at first? the serial number is OM383024. we dont want to just throw parts at it or waste time if there is a good starting point to figure this out.