We have a CRAFTSMAN Riding-Mower-Tractor- model # 917274040. Our lawn mower will start, but after a very short time it starts shutting off like it isn't getting gas.It has the normal engine sound, gets quieter, louder, alternately, before shutting off. My husband has replaced the fuel pump, fuel filter, air filter, and vacuum hose. We were sure the new vacuum hose received today would do the trick, because after we had replaced the fuel pump and used a vacuum hose that wasn't exactly the same as the other, it ran fine for a while. Needless to say, it's getting frustrating. Have we overlooked something that might have been obvious? Thank you for your assistance.