This is a new problem which started the first time I brought the boat out this year. Engine starts just fine. Runs great for about 15 minutes (including idle), then just dies. Already checked the usual suspects, gas, plugs, and fuel line. I know it is getting gas, since I took the cover off, elevated the tank and watched until it quit. I also replaced the membrane in fuel pump which did not fix it. The engine will restart without any problem once it has cooled, but stops after it heats up again. I did notice that before it died the last time, it did billow some exhaust. Any suggestions?