I had the same problem on my 93 altima. Everything I tested had the right reading but I wasn't satisfied. I finally narrowed it down to the IAC or Idle Air Control valve. When I hooked a multimeter up to it, it was fine but again I new it had something to do with the idle. I took it off and the thing was gunked up. I took it apart and used some Throttle body cleaner and put it back on. Just for giggles I also took of my egr valve and cleaned that out carefully with a screwdriver and throttle body cleaner. I started the car and it purrs like a kitten. The IAC is about $130 so I didn't want to replace it. If it's tested it might pass the test but still be gunked up. It is located on the passenger side bottom of the intake and is a pain in the butt to get off. I hope this helps out. Parts for this style nissan are not cheap at all and cleaning a part is a lot cheaper than replacing it.