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. /QUOTE]
As a newer Phaeton owner I can agree with Ron's assessment.
Before I purchased my car last month I read through a lot of the excellent information on this forum. After my research, the "do" item on the top of my list was to replace the batteries if they were original, they were.
I drove the car for a week before I took it in for state inspection to find any issues. I had a variety of things happen such as the nav lock up and other gremlins. Since replacing both batteries with OEM, I've had zero problems.
I have a bit of experience with battery issues. One aircraft I fly is very advanced, probably as high-tech as you can get outside of the military. We have had a variety of phantom issues pop up, mostly before flights during the start-up checks. The manufacturer traced the issue for the fleet to low battery voltage, a number of the controllers in the aircraft apparently don't like low voltage either.
So, being very much a non-expert, it seems logical to me to always start with at least a new comfort battery, especially if yours is original, it's not IF you'll have problems, it's WHEN!