High rev, intermittent P0171 code

The car revs really high for a couple of seconds if I am cruising through town at 25 miles an hour it will jump up to about 3,500 RPM for a few seconds before going back down to about 1,500. It also stays at 3,500 RPM at 65 miles an hour on the freeway I am wondering if this could be the transmission slipping, idle air control valve, or the mass air flow sensor. I have replaced upstream O2 sensor, spark plugs, tube seals, and the valve cover gasket as well. I have checked all the hoses and hard lines but do not see anything wrong with those. Every now and then the car will throw a P0171 code which then goes away on it's own after a couple hours and will return briefly a month later.