2013 QX56 with 145000 miles. Plugs were changed several months ago, and vehicle has been running excellent, up until last week when vehicle ran rough all of a sudden. Check engine light came on and went to local auto parts and code p0300 came up. I've checked each each ignition coil, no visible defects and the ohm readings were consistent. Anyone have any idea what to check next?

When a P0300 shows up suddenly with no other codes, the prime suspect is always a bug in the MAF. All it takes to make the engine run like crap is a piece of lint or butterfly wing, and since Nissans don't generally have MAP sensors for the ECM to cross-check the MAF, misfire codes are usually all you'll get (if any codes at all).