I just had a very similar problem as yours with my 93 maxima, http://community.cartalk.com/discussion/2297944/1993-nissan-maxima-rough-idle-horrible-gas-milage#latest
I replaced several parts at the same time so I cant say for sure which part,or parts, were causing the problems but I suspect that the MAF was the main culprit,because when I disconnect the new MAF, while the car is running,the engine exhibits many of the same symptoms that it had with the old MAF installed.The old MAF "looked" fine so I didnt replace it originally I just cleaned it and put it back on.
The check engine light never came on and there were no codes which made finding the problem more difficult.Anyhow I hope my experience can be of some help to you and shed some light on the problem your having.