Ok, here's the rundown
1997 Maxima SE, 65K miles, Auto, very well maintained: just did the complete 60k tuneup.
Power Mods: Cold Air Intake, Y-pipe, RT Cat

When I start the car and it's cold, I shift into reverse and the RPMs go up and down continuously, between about 300 rpms up to 1500 rpms. But the range gets smaller the longer I sit there. It's as if I was pressing the gas and letting go.
This also happens if I shift into drive and the engine isn't warmed up yet. When it first starts it feels like the car is going to stall if I don't give it some gas.

After the car warms up, I don't have this problem anymore.
Any clues?

Thanks

Idle air control valve or MAFS. Do a search for either.

I would try checking the fuel filters, and the fuel pump itself, if the suggestions that medisonic made don't solve your problem.

I think this takes out the fuel filter and pump as possible canidates. I agree w/ medi that it's somewhere in the cold idle mode somewhere. ie... the iac controller or temp sensor etc..

"""After the car warms up, I don't have this problem anymore."""