I've got an 04' silverado with a vin 1 lb7 and once every couple months it will crank but not fire no matter how long you crank. I replaced the primer pump, fuel lines and fuel filter. It changed nothing. I can prime it up tight with no air and as soon as I crank it I lose the prime. I can prime it repeatedly and same thing over and over again. I go in the house for 15 minutes, come out and it fires right up and runs great for a few months before the same thing all over again. It's starting to get pretty old. Any thoughts?