This sounds like a fault that I and others have had - crankshaft position sensor needs replacing with a new one - cost about £140 for the part and 4-6 hours labour. The CPS works fine when cold and when hot can make it seem like the starter motor is failing on the car but it is actually stopping the car from starting because it is affected by the heat of the warm engine as it fails / breaks down and so when you wait for it to cool down, all becomes ok again.
Have you scanned the car with VCDS? You will get a code come up which helps to confirm it, but I can't remember what code it was. If you ignore it then it will slowly get worse until one day it totally fails like mine did and the car needs to be trailered to the garage to have the CP sensor swapped
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Audi A8 D2 3.7 Quattro Sport 1999 FL - Melange
2nd Place Winner - A8-Freunde Annual German Meet 2012 at Edersee
20" x 9" Bentley GTC Wheels with 385mm W12 Front Discs & Bentley Caliper Covers, Front Lowered on Coilovers, Xenons, Combined DRLs and LED Foglights to modified OEM front fog lamp position, Alcantara/Leather, Full Black Alcantara headlining + pillars and blind shelf, Bespoke Flat Bottomed Sports Steering Wheel, Dark Myrtle Wood, Autodim Mirrors, Bose, Blinds, Digital TV, Ski Hatch, Modified rear exhaust section for extra V8 Burble 
Audi S8 D2 Final Edition 2002, Avus Silver with Red Leather Interior
Tiptronic with Sport mode button, Sony rear entertainment package, Bose sound system, Audi Navigation Plus, 20 inch Final Edition alloys, Full Cat Back Stainless Milltek exhaust, giving a great V8 burble
Audi A8L D2 4.2 Petrol with LPG - Ebony Black
Audi A8L D4 FL 4.0T Petrol twin turbo - Havana Black
Audi A2 1.6 FSi Sport - Dolphin Grey with Votex body kit and Red Leather Interior
and some other A8's A8L's and Mercs
Last edited by Architex_mA8tey; 3rd November 2016 at 01:09 PM.
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