2006 A4 2.0TDi 140 Cutting out

I have bought a 2.0tdi a4 se140 last week, from a vw audi garage the cars really nice but it keeps on cutting out. it first done it the other morning when i was defrosting it it just stopped but started again stright away. Then after setting off for work the other day it completely cut out when doing 30 and wouldnt restart for 20min i taken it to the garage where its under warrenty for 6 months and they plugged in the computer and said it wasnt showing any faults they said they had a few cars recently with dodgy fuel filters so changed it then ran it for a while and said try it then today when at idle it was like some one turned the ignition on and off quickly it died lights came on the dash then it picked back up just before the engine stopped.. I know its under warrenty but has anyone got any ideas? I dont want it to be in and out of the garage all of the time :/

Regards Tom

they have the latest vagcom and bosch computers.. Batterys a thought i havnt heard of them doing that when its on its way out

Right been to garage couple of times recantly they have replaced fuel filter but the problem still persisted they have now tried fuel relay. The car isnt throwing any fault codes off at all so its a bit of trial and error..