Pontiac Repair: pontiac grand am 1999, oxygen sensor, pontiac grand am

Question
okay, i'm no mechanic.  i bought this car and had it for a total of 3 months and this will the 3rd time i'd have it back to the dealer to have it looked at.  the first 2 times they found nothing wrong. it has the 2.4l.  the car will not idle and the service engine soon light comes on. the codes are misfire #1 cylinder, something with the oxygen sensor, and a third code in which i cant remember.  this problem disappears one day and then comes back.  Any idea what's going on here?  if anything to lead the dealer in the right direction. oh,  each time they checked it the car was fine for a month and then it comes back. Other people i have disscussed this with point towards the ignition module.  Why would the dealer be having problems detecting this?  

Answer
i would take the car to a few different mechanics and see what they say.it is difficult to tell when the codes come and go.you could have a bad coil that works sometimes and sometimes under the right conditions misfires.i would have the coiles tested before replacing the module.