go to a nissan dealership and they will attach a consult and be able to tell u if afm, idle, and tps settings have been played around with..
a friend is having idling problems with his gtr, has been traced to the afm's (commoN) but to start with we went to nissan (hornsby) and they picked up the tps was set wrong, as was the valve that controls the idle, these problems showed up on the consult, and they give u readouts of before and after..
it cost $104 and is probably worth doing, after the settings were fixed, throttle response was much improved, and idle/revving generally felt smoother..
if that doesnt fix all your problems, it would probably be your AFM's(well one of them at least!)
good luck!