Hey,
Have received my 32 GTR from japan in last fortnight and have this exact problem. Did not seem to have it when i first got it, but has pretty bad now. Seems to be a totally random thing. SOmetimes drives sweet then does it, other times does it as soon as i start the car, comes and goes all the time. I have found that if, when it does it, I get out and jiggle the wires on the top AFM the idle will cahnge and is now sweet again, only to come back again. Have had both airflow meters run througha diagnostic with no evident problems. Will be getting some spares to test with soon. If you have any luck pls post as i would love to fix this also.