If the idle is dropping below 800 RPM or it feels like it may stall when you come to stop, it could be the Idle Air Control Valve. Otherwise there is a rough running condition related to the fuel system or igniton system.
You should check the air filter for its condition and the air intake tract for leaks or blockage. A vacuum leak could be causing the problem. So, listen to the engine with the hood open and look for a leaking hose or connection.
The fuel injectors may be dirty and need a high quality fuel system cleaner. Try BG44K or Sea Foam. BG44K is harder to find but works better. You may have to go to a service station or the parts department of a dealership. The Sea Foam can be found at many auro parts sotres. Use two cans of it and then give it some time as the deposits it removes will still cause rough runing until they are all settled out into the engine oil. You will want to change the oil after a few hundred miles after the last gas tank with cleaner is used and you have driven it until it has had a chance to clean deposits, aproximately 100 miles.
Other than that, there are a few possibilities tha we have a diagnostic walk through for. It is very good at showing you how things work and has a good logical order to check for problem source in.