'99 Rough idle on cold start- runs perfect after 15 seconds

[txg37x]

I have a 1999.5 QX4 with 196,000. Love it, but I have a rough idle problem that is a little concerning. For the first 15 seconds when starting on a cool (maybe humid day), the engine idles rough and misfires- RPMs under 800. When I push the throttle and get it above 1200, it seems to run fine until I let off and it drops back below 800 again. This does not happen all the time, but very often when cool weather and it hasn't been driven all night. Problem seems worse when the AC is on and becomes more smooth when I turn off AC (maybe because more strain on engine?). After around 15 seconds when it warms up and idle stabilizes over 1200 RPMs, the car goes back to running perfectly. Problem seems directly related to RPMs. No check engine light or codes due to this problem.

I did a tune up with new plugs, wires, fuel filter, and timing belt, and while normal idle is perfect now, the cold startup problem persisted. I have no other issues with the car.

I appreciate your input!

[NismoPick]

I would start with downloading the fsm, then checking the cold idle components.
http://www.nicoclub.com/archives/infini ... nuals.html

[spike753]

After you did your fuel filter did you check the pressure see if psi is in spec? I had a truck that did something similar and it ended up being a fuel pump on its last leg. Of course I waited until it was completely dead and was stuck on the side of the road even when I knew the psi was getting a little lower than spec...

I just found this for myself yesterday. I am going to check mine on the qx4 today hopefully so I don't repeat the truck incident again

[04pathse]

Could it be something as simple as a dirty MAFS???