I agree with David - checking the computer memory is always the first step at diagnosing any computer controlled vehicle. Any stored code will help guide in further diagnosis. Some parts stores (like Autozone) will check the computer memory for free. You can also buy software and cable kits that will allow you to use a laptop computer as a scan tool. Here is the one I have: [http://www.amazon.com/Crescent-OBD2-Multi-Protocol-Diagnostic-Scanner/dp/B001MT0XPK/ref=sr_1_2?ie=UTF8&qid=1341983873&sr=8-2&keywords=usb+obd2+cable|cable] No, I am not affiliated with this in any way.
The software also allows you to read sensor inputs which can help diagnose intermittent problems. Trying to nail down an intermittent problem can be quite frustrating, even for the best mechanics.
Hope this helps,
BILL