I know you've already invested a fair bit in an effort to rectify this dilemma, but how about buying a new battery clamp so you have a good, solid connection? Loose contact with the battery terminals/posts is not a good thing. While in there, I would seriously check your earth contact too and any other connections in the system that are visible in the trunk/boot space. Have you checked connections to the alternator? Sounds to me like you have a faulty contact somewhere and you won't find it until you check for the obvious by pulling, prodding, wriggling and tightening. At least this won't deplete your wallet anymore....

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08/06 build 4.4i Stirling Grey over black