I have a 2007 Mazda 3 2.3 DOHC and I have had it almost 3 months. About a month ago it started having trouble starting. I have changed batteries and starter relay and check the fuel pump everything is working. It will crank but not fire. It will die while driving and I can’t figure out why. I took it to a Mazda dealer and they said nothing is wrong with the car. Any idea what is wrong?

it also has a ruff idle from time to time