High RPMs in cold weather

1996 Chevy Lumina sedan, 3.1, 75k miles. When the weather drops below about 30 degrees, the car drives at a high rpm for the duration of my 20 minute commute to work. I have to ride the brakes to keep from speeding, as the car will slowly accelerate on it's own. The rpms are not erratic, just consistently higher than normal. No tach, so I don't have specific numbers.

This problem first happened last winter, but went away completely after the seasons changed. It's getting cold again, and the problem has returned. No troublecodes are present when hooked up to an OBD2 scanner.