I had a similar issue with my 496. It was under warranty so the dealer fixed it. I didn't troubleshoot (or know how) but apparently there is a low pressure (idle?) and high pressure (running?) fuel pump on the 496. So 2 pumps. My boat would run great at idle or very low rpm's but the minute I hit the throttles it died. They changed the pump and its been great for 2 seasons. Maybe this helps?