2000 Prizm running bad, Please Help?

I have a 2000 Chevy Prizm with 144k miles. The car starts fine on the first crank everytime. My problem is it sounds really bad especially when I put it in gear. Its like it is "bogging it down" or growling. The RPMs drop also. I can drive it without any problem, but I really want to get it fixed. The check engine light is on and is giving the emissions code. I`ve had the pressure test done on the cat. converter and they said it was fine. I changed the spark plugs about a week about and the old ones were fouled. I also cleaned the MAF sensor with MAF sensor cleaner. It was also real dirty. The car ran great,without that growling noise for about 5 days and the engine light went off. I have not changed the plug wires. The engine light has come back on and the car sounds bad again. Please help. It seems like alot of people on the net are having this problem also. I was wondering about maybe new plug wires, new MAF sensor, PCV valve, EGR valve....Thanks