Like they mentioned in the above posts, it could be a number of things and unless you scan for a code, you really won't know for sure, especially since you have an OBD2 compliant system. This means, you need a scanner.
What I can tell you is that with your mileage and model year, the EGR and passages around the throttle body and intake manifold mating area get plugged with carbon. They need to be cleaned every 30K miles or sooner to keep this part of your emission system running optimally. Sometimes the O2's and/or CAT's go bad, especially if your engine has been running rich for a while. Anyhow, one quick and cheap check on your EGR is to pull vacuum on the valve itself. Idle your car and remove the top vacuum tube from the valve. Pull some vacuum on the valve using a small hose and see if the engine begins to stumble or run rough. If it does not do anything, then you are plugged up and that could be part of your problem. Other than that, you need diagnostic equipment cause you will go nuts trying to figure this one out.
