I have found that most intermittant problems with automotive electronic devices is the solder joints where the plug-in socket or pins is soldered to the board. This is usually caused by the weight of the cable bouncing around as you drive. If you look very close using a magnifying glass, you will see hair line crackes in the joint. They usually look like a circle around the pin. Also look at the pins around any heavy componts like relays, etc. They will also crack. Sometimes on double layered boards the crack will be hidden from view by the componts. Carefully resolder all the joints mentioned above even if you don't see anything. This trick has fixed many expensive electronic parts that usually get replaced.

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84 500 SEL (307,xxx miles)