Grand Am mystery problem
 

I have a 99 Grand Am Gt 3.4l .....Everything and I mean EVERYTHING from heads up including heads all fuel components, all sensors, computer has been replaced but still has a low rpm hiccup ...not bad when car is cold but get progressively worse as warms up ...Car idles fine takes fuel fine in park or neutral ....when car starts out stumbles wont take fuel sputters almost stalls release throttle goes back to smooth idle then ease on throttle accelerates sometimes does several times ....Up to around 2000 rpms then it's fine runs great as long as rpms up ...once back below that starts stumbling again ...If gear down ( out of OD or in 2nd gear does better ) ...there is no check engine light ....I need fresh ideas hate to sell car have lot of money in it by this time ...but at sits and knowledge end .