High-Probability Suspects:
* Crankshaft Position Sensor (CKP): This sensor tells the computer where the engine is in its cycle. A failing CKP sensor can cause intermittent starting, stalling, rough running, and fluctuating idle. The fact it runs fine *sometimes* strongly suggests this. This is a relatively inexpensive part to replace.
* Mass Airflow Sensor (MAF): This sensor measures the amount of air entering the engine. A dirty or faulty MAF sensor can lead to a lean or rich fuel mixture, causing stalling, rough running, and a check engine light. Cleaning the MAF sensor (with MAF sensor cleaner, *not* general purpose cleaner) is a good first step, but replacement might be necessary.
* Idle Air Control (IAC) Valve: This valve regulates airflow at idle. A dirty or faulty IAC valve can cause fluctuating idle speeds, stalling, and rough running. Cleaning it is worth trying, but it's a relatively inexpensive part to replace if cleaning doesn't help.
* Fuel Pressure Regulator: This regulates fuel pressure to the injectors. A failing regulator can cause inconsistent fuel delivery, leading to the symptoms you describe.
* Faulty Fuel Pump (Less Likely, but possible): A failing fuel pump might initially provide enough pressure to start, but then struggle to maintain it, especially under load or after extended use.
Less Likely, but Possible:
* Vacuum Leaks: Leaks in the vacuum system can disrupt the engine's idle and overall performance.
* Ignition System Problems: Worn spark plugs, wires, or a distributor (if equipped) can cause misfires and intermittent running issues.
* Computer (PCM) Issues: While less likely, a failing PCM can cause erratic behavior.
Troubleshooting Steps:
1. Check the Check Engine Light: Get the diagnostic trouble codes (DTCs) from the car's onboard computer. An auto parts store usually provides this service for free. The codes will narrow down the possibilities significantly.
2. Inspect the MAF Sensor: Carefully remove the MAF sensor and clean it with MAF sensor cleaner. Let it completely dry before reinstalling.
3. Inspect the IAC Valve: Try cleaning the IAC valve (often involves removing and carefully cleaning the valve with carb cleaner or electronic parts cleaner).
4. Check for Vacuum Leaks: Visually inspect all vacuum lines for cracks or disconnections.
Important Note: If you are not comfortable working on your car's engine, take it to a qualified mechanic. Improper diagnosis or repair can lead to further damage. The intermittent nature of the problem makes it difficult to diagnose without the proper tools and experience. The DTC codes are crucial to getting this fixed effectively.
