Exactly as described by plob above.Can you put up a link or describe the problems- the heading is a bit broad. However I will say that i have not had tram lining issues, but 4wd is not going to be the same as front wheel drive that i think is the system on the 1.8T. I have experienced it before on powerful front wheel drive cars and it is partly tyre affected, also tyre pressures and alignment of the wheels are all worth checking.
I have run all variations of tyre pressures and what I running now just under 30 psi is by far the best. I picked the car up from dealer when they fitted new wheels and tyres and they had 36psi in all wheels and it really was undriveable. The cars tendancy to jump off line was exagerated As far as I can see having driven 5 different A5s at this stage, it is a characteristic of the car that is showing up strongly on the roads im using but some cars are worse than others. That may be tyre related.micdw - sorry to read about your problems.
Although I have a completely differnt car (BMW 525 MSport) I have experienced a long running saga with the same symptoms.
My car at different times throughout it's life has exhibited an alarming tendency to tramline and pull me accross a camber. Originally I read the BMW forums that all pointed to worn bushes as a known cause of this. Although the car only had 10k on the clock when I acquired it I badgered my dealer constantly to check the bushes / tracking / suspension etc. but all to no avail and the dealers only comment every time was -"tyres". With my tyres only part worn I was not happy with this answer and blamed it on the fact they were runflats.
When I replaced my tyres I noticed a massive improvement with the problem but very shortly into their life it seemend to return.
On the verge of getting rid of the car because of this problem, I brfriended my local tyre dealer for some help. His answer was "uneven wear due to incorrect (too low) tyre pressures" The combination of run flat sidewalls and low tyre pressure led to the pulling action when crossing a camber. I know that you don't use runflats but he did explain that low profile tyres have hard sidewalls and will exhibit similar tendencies at low pressures. I was sceptical but resolved to check my tyre pressures every week and after 75,000 miles of doing this my cars handling has improved tremendously and the only time i feel any tramlining is when my tyres near replacement.
I had read your post earlier and presumed that your garage would have checked pressures etc. but I have just read your other post at http://www.a5oc.com/forums/showthread.php/19-y-tyre-3618/index.html where you confirm that you run the pressures at 30psi max. I do think that this may be a large part of your problem so would urge you to try this prior to ditching a car that most people have no problem with (as I nearly did).
Cheers and good luck.