If your car has over 40,000 miles its possible that the sparkplugs need replacement. Although the owner's manual and dealer service centers will tell you that the spark plugs will last 100,000 miles. Experience has proven this to be false in some cases. Worn spark plugs, or spark plug wires will definately casue a rough idle.
Another possibility is a vacuum leak. Check the vacuum lines under the hood, especially the large tube that leads from the ntake manifold to the brake booster. Some people have found this tube to be worn through.
The cold weather has an efect on water in the fuel. Gasoline will absorb and hold some water in suspension, but this ammount decreases with drops in temperature and the water precipates out and settles to the bottom of the tank and can be sucked up and casue rough running, sputtering and even stalling. Try using a bottle of dry gas in your next fill-up. Even if its not needed the dry gas won't hurt.
Another situation is dirty fuel injectors. They may have accumulated a small deposit on one or more injector of the heads which can change the spray pattern. Also try running a bottle of fuel injector cleaner in you next tank of gas.
