My 2003 s2k usually idles around 900 rpm, after i go driving the idle drops down to roughly 400 rpm and sometimes stalls

 

I think you need to clean your idle control valve. There is a DIY on the forum.

 

Check voltage at TPS. Closed throttle should be .5v and WOT should be 4.5V input to the PCM. Possible it can be your issue along with IAC.

 

Thanks guys i will check the idle control valve tomorow

 

Also check the map sensor. There's a little air channel that runs between the throttle body and intake manifold that get clogged and impinges flow to the map sensor, and can cause issues regulating idle after removing throttle. Just be careful when unbolting the TB that you don't harm the gasket.

 

All s2ki do the same, ap1 & ap2, new or older!