It could be many different things.
Air filter? That's commonly overlooked and easy to check/replace.
I'd also say get the fuel filter replaced too if it hasn't been done recently.
If you haven't already, look carefully at the connections and vacuum lines under the hood. One cracked or disconnected vacuum line or loose electrical connection can do a lot.
If none of these help or you have a "check engine" light on, it may be something hard to figure out on your own.
For example: a bad throttle position sensor (TPS) can cause all kinds of idle issues; a fuel pump that's not putting out enough pressure may cause the injectors to misfire; a bad oxygen sensor will cause improper fuel mixture (rough run). Please note that I am NOT recommending you run out and have these items replaced, these were just examples of what could be contributing to your issue. As you can imagine, "guessing" can get very expensive.
