2004 Mazda 3 Sputtering

My car started sputtering and lunging months ago on my way to work. I drive about 100 miles to work mostly on the interstate. It was faint on the interstate, then much worse on the side roads. I assumed it might be the spark plugs or the fuel injector, so I replaced the spark plugs before heading back home and it fixed the issue. About a month later the same problem returned. Autozone only had the cheapest spark plugs the first time and said it was probably just an issue of quality, so I changed them again for the NGK iridium's. The problem was fixed. Well it started again, about a month ago, but not as bad as before.

Well, before I could get it looked at by a mechanic, it fixed itself- even the check engine light went off. It's been fine since then, until yesterday morning. Started sputtering again after I got off the interstate, but when I drove back home this morning, it was fine. The check engine light came on yesterday morning, then when the problem stopped, it went off.

Any ideas as to why or how it is so messed up and then fixes itself?

Thanks for any help,

Kory