little help here?

so here's the deal;

i just got a g20 from my brother and its pretty tight, few little problems and one that has me stumped. when the car is warmed up it will stall out. it only occurs when the car is completeley stopped, be it in neutral or in 1st gear. the rpm's will start to drop under a 1000 then pop up then drop down to 200 or so and then pop up and eventually on the downstroke the car stalls out. so i replaced the plugs cause they were due, as was the fuel filter, i also did a fuel injector cleaning. it was still doing it. so i bumped up the idle a bit, it idles at about 1400 now and i dont get the stalling but it doesn't seem like a lasting solution. i'm thinking maybe fuel pump or o2 sensor. anyone seen this before? any advice would be apreciated, it has me somewhat baffled and i'd hate to bring it somewhere, you know how expensive the repairs are at luxury car dealers. thanks

also cleaned out the throttle body