Sounds like you might need your injectors cleaned up.
Have you tried any injection cleaners ??
Seafoam works great!
Gm does recomend that the fuel system be cleaned professionaly about every 30-40K miles,
as these injectors and the lines get build up and can cause the injectors to clog/stick.
Might also check your IAC.
Idle Air Control(IAC) Testing Replacing for 99+
and Welcome to AF!