I've had this 02 bonneville for about 4 months and did a complete tune-up such as plugs & wires, egr valve, all filters including fuel and oil change. I replaced a bad water pump and installed new thermostat so the running temp is spec and engine was running great until recently the car idles real rough on cold starts I can hear all the plastic vibrating until the engine reaches running temp and it smothes out and all is well again but even if I drive the car cold it drives fine its just the idle that is rough when cold. I'm no mechanic but I try what you tell me because I hate cold starting this thing.

Thank you any help be much appreciated.
2002 Pontiac Bonneville SE 101K