Actually after hearing you say it has fixed itself,
It sounds exactly like what happened to my old silvia which had an rb20det in it, it used to run exactly how you described every now and again and then it would fix itself, but it did this on and off until it just constantly was buggered and the problem was a buggered AFM what had actually happened was the mother board or whatever it is inside the square bit of the AFM actaully cracked, and believe it or not it was actually caused from constant bumps because i had such harsh suspension in it.
Now dont take this the wrong way i am definately not having a go at you about the way you have your suspension as i really like the way your car looks.... But i am telling you the truth when i say that my AFM did stuff up because of how harsh and bumpy it was in the car.
So i would say get your AFM checked out, although i know you are putting in a Z32 AFM so then again you may not need to.