Cylinder misfire, o2 sensor..?

Ok, well my friend has an 04' Eclipse GS with 17k miles on it, has taken good care of the car ect. Anyways, since my friend just put on a ceramic header on his car it was idleing really high when he jjust started it, it went up to about 1800 rpms, then it would go down below 500 and eventually DIE.....and the service engine soon light comes on, so he took it to advanced auto and got a read on it, it said it needed an O2 sensor, so my friend gets that put on, the car runs good for most the day, and then we get out of our movie and it happens again.....The car also is jolty when its being driven...So we take it BACK to advanced auto and this time it reads as one of the cylinders mis firing, my friends 99 eclipse got diagnosed with the same thing, and all he had to do was replace the spark plugs. Do you think its the ceramic header that is causing it, thats the only thing we can come up with since it never happened until the header was put on, any information will be MUCH appreciated.

uhmmmmm...just so this doesn't go unanswered, I HAVE NOT A CLUE, but my theory is that the headers somehow messed w/ the engine in a negative aspect

which brand of header did he get
did everything go back in correctly? the majority of my friend's problems on his car is we couldn't get everything back 100%, things broke, had to be replaced...really sucked,

also did you reset the ECU? it sux i kno, but if you don't reset it, it'll remain in the state it was before you altered it and it will never work right, just unplug the battery for 10 min. or so