2001 Mazda 626 - 4cyl

I am having problems starting my Mazda, it has 72,000 miles on it. After the car has been sitting for a bit it is very difficult to turn over. If it hasn't been sitting very long then it starts easily. Also when you are idling at a stop light it sounds like it may die. We've plugged it in and they said that the 3rd Cylinder wasn't firing. We've changed the air filter, fuel filter, spark plugs, and coil. Not sure what else to try.