2004 Sentra-Rough Idle

I bought a 2004 Sentra July 2004 (demo car), the next month it developed a rough idle while in all gears except for park and neutral. I took it in and the dealership said a motor mount was broke, they replaced that, I picked it up and started home and it was still doing it, took it back, they said I had a flat spot on my tire, replaced tires, still doing it. I've taken the car back numerous times since only to have the standard "can't duplicate the problem or that's normal" excuse. I did finally have one mechanic admit he felt the problem and he adjusted the timing, reset a computer and made a few other adjustments that they wouldn't tell me which ones. (I also had an O2 sensor replaced a few months ago.) They are not capable of fixing this obviously so after calling Nissan consumer affairs, they suggest I take it to a dealership in another town. Everyone who rides in or drives the car feels the roughness. Any suggestions of what it could be before I go to the other dealership (I am a female, so they assume I don't know anything and I would like a little foreknowledge)