the only thing left is lack of or intermittent pressure, which could be the valve in the steering rack or the pump itself, the rack is a nightmare of a job. The pump makes pressure obviously but a valve at the end of the steering column opens on one side or the other depending on direction the wheel is turned and allows pressurised oil to push the rack in the direction you want to turn, decreasing the effort. The valve can jam or stick, and is probably more likely than the pump though that can fail too, but is usually noisy. My B5 valve used to stick shut first thing, no power steering, then after a while of running it would give a soft click and the steering would work, then stop again later.
Best to contact a power steering specialist and get it checked out. Audi racks are prone to leaks, and the engine undertray can disguise where its coming from. I had the rack seals, valve and pump go bad on my b5 and it was a real shitter honestly mate. The previous owner had got the leak first, and was topping up the ps with red atf - the wrong kind of fluid (has to be audi green) and it rotted the seals. The pump ran dry and the valve started sticking, just got worse and worse.
The engine and box have to come out and the rack itself costs a fortune, also theres a coupling in the steering column that if it slips when the rack is being disconnected (and its easy to do this) then its a whole new steering column too. It was the first job on AutoData that my local garage had ever seen come up in red! It warns about being an excessive labour job.....
I ended up scrapping the car as the cost of the job outweighed its value but then it was an old car.
