I have a Volvo penta 5.0 sx drive and I am having a water ingestion problem. Water is being ingested into all cylinders, both sides. Has happened twice now when I come off plane, idle and then after a minute or so it will stall and hydrolock. Have had the boat for 3 years now this has never happened. Then i pull all the plugs, blow out the water and get it running again. Can’t imagine both risers are bad? Or both manifolds, or both head gaskets? Is there an exhaust flapper somewhere? I know Volvo got rid of the flapper in the Y pipe in the 90s? This is a 2006. This problem just started happening this year. Any help would be greatly appreciated