Problems With Idle

When they replaced the alternator the battery was most likely disconnected which caused the cars computer to loose it's memory. The computer will learn idle strategy over time while you drive it along with other various parameters. Your car most likely has some mileage on it so it has accumulated some wear and sludge on the electronic throttle body. Cars with higher mileage on them suffer this problem worse. You can keep driving the car and it will eventually get better, or you can clean the throttle body (disconnect the battery for at least a minute when you are done), or you can return the car to the dealer. Cleaning the throttle body may or may not fix it. It usually will. Be forewarned though that cleaning the throttle body is not covered by warranty. The dealership will try to sell you either a throttle body cleaning service or replacement of the throttle body itself.

Seafoam has a throttle body cleaning 'kit' with a specially designed tube that attaches to the lip of the throttle body and provides a good point to spray. Works good on a few cars that I tried it on....