I had similar and it ended up being the vacuum hose from the top of the EGR to one of the controller sensors.
I've ended up replacing all of the vacuum control units N18 and the other one, but not the N75 yet. I've also replaced all the vacuum line.
This is on a 2001 1.9 TDi B6
IIRC the front anti roll bar is the same across the range? The rear rollbar is different and I can highly recommend fitting the RS4 rear ARB.
For the front, I assume you mean the small linkages? In which case I've replaced both on mine, cost about £30 each side IIRC, you need to put both...
I've never liked the idea of those suction pumps, I much prefer just getting underneath the car and making sure all the crap comes out.
I also tend to pour about half a litre of clean oil through to make sure too.
Almost a snap here (1.9 TDi, 51, 85k).
I'm using Valvoline Durablend now, 5 litres for £20 and it meets the VW 505.01 spec for fixed interval servicing.
I'm changing oil at 6k miles, I like it, it's a good price and it works well with this engine.
It means the reserve battery in the alarm siren is running low, needs replacing.
Unfortunately Audi only sell them as complete units and they cost around £90.
I cut mine open with a dremel and soldered in a new battery pack as per some instructions on a TT forum which I'll try and find and...