I had that same problem in February 2004. Dealership replaced upper and lower intake manifold (UIM/LIM) gaskets - four total - ran fine after that. It was $306 with labor. There is probably an easier fix (IAC?) as Tony suggested .. after all, this was the dealership .. just relaying my experience.

Matthew
2000 Nissan Maxima SE
Sold: 98.5 Contour SE V6 ATX