i had this same problem, and it got alot better when i changed the plugs and cleaned all electrical connectors ie AFM, TPS, Coils and Ignition module...
but yeh it always goes away once warmed up, but got so bad i couldnt even drive the car when it was cold! the exhaust had a much louder drone to it and there was no throttle repsonse around 2000rpm. sound like the same problem youre having - im not sure exactly what fixed mine, but thats what i did...