stutters and stalls

This sounds pretty urgent, in that it could be that your Engine Computer could be losing a connection to a power or ground source, like a loose battery connection etc or that the Computer itself is having problems. I would get this checked out. Who checked the codes, if it was a parts store, that is not sufficient because there are several computers to check for codes and most parts stores just have a REALLY basic, hand held scan too. You need a Professional level tool with a trained Technician doing a whole systems scan, not just for a check engine light code. You could have a defect in your Instrument Panel, that can cause a car to die out, yet there will never be any Check Engine Light Codes. http://repairpal.com/nissan-repair-in-simi-valley-california