[oh one ls1 SS]

when i first start my car (when its cold) and i start to drive it the bitch bucks/hesitates like mother and when i come to a stop the idle drops to the 300-500 range but after about 10 mins of driveing it its pertty much normal. i was throwing some 02 codes so i swapped them out lastweek. and it is still hesitating n whatnot. but now,after its warmed up, and i drive it hard it loses power about 3Kish rpm's. i took it to my mechanic and he said that its running lean but he doesnt know why yet. its so slow now that i cant even chirp in 2nd and can barley get the tires to spin. HELP!!! any suggestions would be great