99 santera 325 indmar. Just bought boat so I don’t know the history. When it’s towing around 10 to 15mph will run perfect for a few minutes and all of a sudden just looses power and starts shuddering and backfires but will idle. Turn off for a minute or two and works fine for a little bit and does it again . Runs great at idle or just above, and will run perfect all day long going 20 mph plus. When it happens if feels like the fuel pump is shutting off and your running on fumes. I have searched all over but mostly what I find is boats that will not run good over certain rpm.