This is a strange one. Usually when I turn my ignition key the car immediately starts. But then, probably 3% of the time when I try to start my wagon, it won't. It seems to have electricity--the engine turns over beautifully again and again and again. I try pumping my gas pedal, then holding it down, but to no avail. I recently learned at these times that if I roll the car down a hill and pop the clutch, it starts beautifully, so it must also have gas. What is missing? Please advise!