Why is my car running badly after I disconnect the battery then reconnect it?

I have a 2003 Mitsubishi Lancer ES, 4dr, 2 liter 4 cylinder, 5spd. And my service engine soon light came on about 5 months ago. I figured it might be an oxygen sensor problem, so I disconnected the battery for 30 minutes and then reconnected it and started the car. The service engine soon light turned off and remained off, but for the next 2 days my car would keep stalling if I were to drive it and come to a stop (at a red light or stop sign). I would have to keep my foot on the gas pedal to make sure the engine wouldnt stall. After 2 days it didnt do it again, and everything was fine. Until yesterday.I was just driving on the freeway and my service engine light came on again. So I repeated the same process (disconnected the battery for 30 min and then reconnected it) and when I started it up the engine raced up to 4000 rpm, like it was flooded with gas or I started it with my foot on the pedal (which my foot was not on the gas pedal when I started it). But now my car is running badly again, and im wondering if you guys have heard of this problem before. Anyway if you could email me back ASAP it would be greatly appreciated. THANKS!

-Sincerly,
Daniel Gossner