I'm having issues with my 2012 Hyundai Santa Fe. I have the 2.4L engine and having some issues with the vehicle. The car idles low at a light right about 500 rpm. The rpms drop when you place it when gear. The car jerks and bucks while driving. The feels like its stumbling when you slow down. I do have a custom exhaust, just the muffler. What would cause the car to act this way? The dealer could not duplicate the problem. I need assistance on this matter.