The "rough idle" symptom encompasses many systems in the vehicle, which must all be checked. Is the "Check Engine" light on? Have you had the codes read? (Codes can be read and interpreted on your Cadillac using the air conditioning controls, the procedure for which can be found online using any search engine.)
Generally, especially on a car as old as yours, you will most likely need a MAJOR TUNE-UP. You will need to check all of the following:
- Condition of the fuel in the gas tank (especially if the car sat dormant for six months or more);
- Operation of the idle air bypass (Idle Air Controller);
- Cleanliness of the throttle body and its components;
- Condition of the fuel delivery system, including the fuel filter, pressure regulator, electric fuel pump, and injectors (this is the most likely suspect here);
- Condition of the coil packs or distributor cap and rotor (I don't remember which your car has);
- Condition of the spark plug wires;
- CONDITION OF THE SPARK PLUGS (obvious);
- Operation of the ECU (or whatever you prefer to call the computer that controls your engine);
- Presence of any vacuum leaks or cracks in the intake manifold.
START BY REPLACING YOUR FUEL FILTER, then check for proper fuel pressure at the fuel rail. Replace the pressure regulator as necessary and, if you are not getting enough fuel pressure, it is possible that rust and debris in your fuel tank has clogged the intake strainer and may have damaged your electric fuel pump. Once you have eliminated fuel delivery issues, continue by examining your ignition system (plugs, wires, etc.).
Best of luck to you!