a4 quattro 19 tdi 2003

[craigieboy]

rear end sometimes feels twitchy when cornering as if the back end is sliding what could the problem be

[Dr G]

Presumably you've checked tyres, pressures and alignment already?

quattro cars are very tyre sensitive

[istoo]

As above.
Also, and this is purely personal, as rule I change all my tyres at once on the Quattro(s). But i do a tyre rotation at half wear, fronts/backs. I found the Falkens I used to use on the coupe were hopeless in the wet and made the back step out in mild damp. Switched to Pirelli P Zeros afterwards and then have never bought a budget tyre since.