One other thing to check on the car before spending more money is the cables. Make sure the plus and ground cables have solid connections and no significant corrosion. I once had a Golf where the gremlin turned out to be the plus cable corroded up inside the insulation (voltage drop between battery and alternator ends provided the clue). And I know we all have discovered loose terminals even when we were certain it just couldn't be the case.

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Prost!