1997 Buick Lesabre Rising RPMs When Started Cold

When I start the car in the cold (under 50 degrees or so), the car turns and starts fine, but then right after the rpms start to increase. It goes up to about 2000 rpms in under thirty seconds of the car running. This morning I let it run for five minutes, and when I came back to the car, it was at 3500 rpms. I turned it off, and then turned it back on, and the rpms rose again to 2000. When I put the car in drive every morning at about 2000 rpms, the car drops to around 1500 (the usual would be 1000 or less) and it continues to stay that high, and pushes against the brakes, until about 100 meters away, when I step on the brakes and get the car to come to a complete stop, the rpms will go back to normal (under a thousand). I need to find out what this is, because it is starting to get very cold in the morning, and I don't want to be driving the car without letting it warm up.