I am having intermittent stalling problems with my 1991 15 hp Evinrude model E15EEIA. I have changed plugs, fuel pump, the fuel line between the fuel pump and carb, vacuum line, and fuel tank. I have also added a fuel filter to the fuel line. The stalling happens on both a warm or cold engine. The engine may run fine for hours then stall or it may run for 5 minutes then stall. Stalls at low speeds and moderate speeds, I have not experienced the problem at wide open throttle however I do not run it wide open often. Once the engines stalls it is usually difficult to restart however it generally will start after enough cranking. When it stalls the prime bulb is still rather firm but not hard. Other than the stalling the engine runs well. Any suggestions as to what I should try next?