Cylinder Misfire Code on 2004 Nissan Sentra

Hello-I recently purchased a 2004 Nissan Sentra (39,140 miles). I've been driving it about 3 months (not even 2,000 miles) and I went to start it and it hesitated. It started the second time, but was very rough and idling. The service engone soon light came on. I brought it to a local Nissan dealer who diagnosed it with a code 3 Cylinder Misfire. They suggested a 30 k service ($420) and 2 repairs: 1. Throttle Body Fuel Injection service and 2. Idle AIrlodging. Needles to say, I had the two done and $350 later, I picked up the car. I started the car a couple hours later and it had a rough start again. Within minutes the Engine light was back. I brought it back to the Nissan dealer who diagnosed the same code (cylinder misfire). The technician "recommended" a air sensor replacement which would run about $300, but could not guarentee that would solve the problem. Well it was replaced, along w/spark plugs and coil. I went to pick it up and it wouldn't start? If the dealership cannot find the problem. Where else can I turn? Any suggestions?