Is it the coilpack?

Ok I have a 2000 Elantra. The car was running fine except for using alot more gas than normal lately. Well today while at a red light it started jerking like it was trying to move forward and the rpm's were bouncing like crazy. Well as I was trying to make my way home the car all of sudden jerked and started acting like it was about to die and the check engine light came on. Well when I got the car home we checked all the fluids there fine and because it sounds like it is missing we replaced the spark plugs. This didn't fix the problem. When you first start the car the rpm's drop to like 1 1/2 and its jerky and it stays that way until you get up to about 25-30 mph. We tried to take it to Advance so they could use the tester on it but they said the hookups are different and wont fit my car. The man there said it was probably the coil pack or the oxygen sensor. How can I find out which one it is and has anyone else had this problem.

uhhh get the computer scanned and repost, any obdII complaint tool will retrieve the code.