Rough Or Unstable Idle

Ensure ECM terminal voltages are okay. See PIN VOLTAGE CHARTS article.
Use stethoscope to verify fuel injector operating noise.
Check condition of spark plugs.
Check air filter element.
Ensure fuel system pressure is correct.
Ensure mass airflow sensor moves smoothly from fully closed to fully open position.
Check for air intake leaks.
Check engine compression.
Check ignition timing.
Check PCV valve.
Check engine coolant temperature sensor for correct resistance.
Check Idle Speed Control (ISC) solenoid valve. See SYSTEM/COMPONENT TESTS article.
Check for cracks and poor connections (vacuum leaks) at airflow meter.
Check oxygen sensor for fluctuating 0.1-1.0 volt output while increasing and decreasing engine RPM.
Verify Throttle Position (TP) sensor has correct adjustment and resistance value. See ADJUSTMENTS article.
Check purge control solenoid valve function.
Check for poor fuel quality.