idle prob. and milage

[Zreed]

i purchased a 97 240sx se not even a month ago..its right at 1001600 miles to the motor...the service engine light came on..so i did the usually and had new plugs and a oil change...it started burning gas..(service light still on)..so i took it by autozone and let them run a test to see why the service light was on ...it came to show a code 420,,,which them man said to be the O2 sensor on the exhaust manifold..so i replaced it...but even before and still now...it has a idle problem sitting in park and at stop lights...but no lost in power or speed..it tends to jump from 800 to 6 are 500 rpm when it do this...and my girlfriend gets better milage with he xterra...it only does this when i am stopped are in park...im open for any suggestions....its a 2.4 L DOHC all stock..

[Dattebayo]

There is tons of information on idle problems here.

So we have a button so we do not have to answer the same questions over and over.

Thanks!