Weird Total Power loss - Then Back On in my E39 540

Hey Everyone, I have a 2003 540ia with a strange electrical issue, can anyone shed a little light on the problem?
Here are my symptoms: Every so often (ie: once per week) my 540 has a
total loss of power. Even the orange light next to the "P" is not on. Absolutely no power, nothing, nada! Several times I have arrived at my parked car to find the doors unlocked (luckily I park in the garage), and once it happened while I was driving (l pulled over quickly). After a moment or two the power is back on 100%, like nothing ever happened..? WTF?
In an effort to determine what the hell was going on, I opened the rear battery compartment and "jiggled" the harness to the battery. This seemed to cause the power to go on and off, and explains why the power comes back on as I close the door (the force jiggles the harness). However, I have inspected the harness and battery connections, and everything seems to be fine...?
I assume there is something I am missing in the harness itself, but maybe I'm missing a bigger issue. Could the battery casue this type of symptoms? What about a fuse or other electrical gremlin? Has anyone else experienced a similar problem?
Also, my air conditioner cuts on and off intermitently (all other power remains on). This might be a completely different issue, but I wanted to include it just in case it is relevant to the power loss.
Thanks everyone!
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I had this exact problem once. I took off the battery connections, cleaned them to within an inch of their lives, put them back on good and tight and the problem never happened again.
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You are pretty close already. Either the battery terminals are loose / corroded or the earth strap [connection] is.

Just likely to be the thermostat.

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Could be a fuseable link - known to give problems on some BMWs.
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Thanks Dave, Can you tell me what a "fuseable link" is? I plan to clean and "reconnect" the battery harness and wires at some point today. If that doesn't work, I need to try other options...
- Ben
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It's an inline 'lump' in some cable or cables close to the battery connection - designed to protect from a fire in event of a collision, etc, and they've been known to go intermittent.
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Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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Regarding the A/C problem, I recently had a problem with similar symptoms. The blower fan would cut off by itself while the rest of the A/C unit continued working and cooling. The fan would come on and off intermittently. The problem was the "final stage" module for the fan. (Around $84.00 + labor). Not sure if you're experiencing the same problem.

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