I don't know if your problem is battery related or not, but as you read through the forum you will find countless stories where electrical gremlins of all sorts have caused Phaeton owners and their mechanics to spend oodles of time and money chasing their tails, only to ultimately discover that simply replacing the left ("comfort") battery cured all ills. It's a few hundred dollars, and it's so critical to the enjoyment and reliablility of the Phaeton, that I would suggest if there's the slightest chance it's the culprit that you buy a new OEM one. Also, you might want to read these threads to gain a better understanding of this crucial component:
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