I had the same issue with my boat. The boat ran great at first and later in the day it wouldn't. I checked everything fuel related and I was about to give up when I found the problem. Are you ready for this? After spending a bunch of time and effort and money it turned out to be the fuel line. Yes the fuel line. The line coming from the tank under the floor was left to long and installed with a large loop maybe a foot or more in diameter. When the line was cold it was rigid and wouldn't colapse. As the boat got warmer the line got more flexable and would get sucked closed causing a restriction as the loop closed in. I removed the loop from the line and all is well. FYI...38 Fountain. It's worth a look.