Try putting a can of carb. cleaner (liquid form Gumout etc..not heat, carb. cleaner) in with your next fill up, the refineries are dumping toxic waste into the gas as "detergent" and it is fouling the injectors, seen it about 10 times in the last year. Also check your oxygen sensor, not too pricey of a replacement, look at air filter..unless it is really bad probably not the problem. Also replace fuel filter. Get on Nissan forums to help with diagnosis..