Hi Hoskins and welcome to the forum.
On the subject of transmissions, you should get a fault code reading done and see what it says. The damp Ian mentions in the ECU/TCU housing could be a factor so having a look would be a good start.
Do you have VagCom? (see the transmissions sticky for more info)
Swapping transmissions is a possiblity. If you can't find the right code transmission another transmission with a different code could do, but you will probably have to swap the rear final drive as well and maybe even the TCU.
Give Ben at AV Performance a ring. He is a preferred supplier to the A8oc and writes for and advertises in Audi Driver. He is THE Guru on maintaining these cars.
As for spending money on these beauties if there is nothing intrinsically wrong with the rest of the car and you look at it in 'Total Cost of Ownership' terms then £1800 on a transmission that will last you another 150k is not that big an investment. You could buy a brand new Ford!!!!!!
Sorry, only joking! 
I have heard of the 4.2 doing 400K  , and stand fast the electrics everything else is at competent mechanic DIY level.
__________________
Conan (the Librarian)
Watch it if you dare
Currently,
2000 S8, Merlin Purple, Leather Pack 1, Winter Pack 1, SPAX adjustable suspension, AP Racing 6 pot front caliper, 385mm fully floating disks, EBC Yellow Stuff Pads, Black Diamond grooved rear disks and predetor pads, D2 Doctors Brake Block.
1997 4.2 A8SQ - the Projekt. 12 years and ongoing!
1997 4.2 A8SQ - Ming Blue. Restoration sat waiting; saved from the Scrapyard
2003 C5 Allroad 4.2Q - Cobolt Blue. Towing car. Laid up awaiting manual conversion.
2005 C5 Allroad 2.5TDi FE - Grey - Workhorse.
2015 Q3QS SLine - Daytona Grey. Lots of extras! Long Haired Admirals cutter.
Previously
2003 C5 A6 1.9TDI FWD - Loaner from D2 Doctor Lifex
1997 S8 - Ming Blue. Loaned out and written off. I loved that car. My first engine swap after cambelt failure.
1996 4.2 A8SQ - Ming Blue. 178k on a cambelt! Trader's 8. Delivered to A8Parts at Lifex.
2003 2.8 A8SQ - powder blueish. Fill in for a while. Sold on.
1998 1.8T A4 Avant FWD. Ming Blue (there's a theme here), PEX for the 97 S8.
And a list of non VAG going back 40 years before I saw the light. 
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