Rough Idle when I start the car

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What causes a rough idle for a minute or less when you start the car? My mechanic replaced the spark plugs, spark plug wires and one coil pack. This did not help. My mechanic cleaned the injectors and this did not fix it. The check engine light has come on three different times when it idles rough long enough. My 1999 Buick Century has 98,000 miles.