Well, this is the original battery so I would start from changing the battery because when mine died one morning with no warning at all it was the day I really was in the rush and needed the car.
Yes, I have this battery fitted in my S8 at present 
Same size, same spec apart more amps, 920 instead of 850.
I fitted Bosch S5 to A8 and had no issue either, but this time I wanted same brand (Varta) for whatever reason which I can't explain 
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2006 D3 S8 5.2 V10
Ceramic Brakes, Bang&Olufsen, DTV, DAB, Rear Camera, BT, TPMS, Soft Close, Auto Boot, AMI , 4 Zone AC, Home Link, All Electric Blinds, Fingerprint, Ski Hatch, Black Extended Leather, Heated Sport Seats, Alacantra, Carbon Trim inc Aluminium Pack 2.
2015 A3 S line 1.4 TFSI - for the missus to keep her away from my car.
Standard S-line equipment, no extras.
2006 D3 A8 3.7 V8 - SOLD but not forgotten
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