A3 cutting out 1.8t also revs drop when idling

Hi

I have an A3 S reg 1.8t. When starting from cold/reversing the revs can fluctuate between 1000rpm all the way down to almost stalling - 99% of the time it picks up but does occasionally cut out

I thought the this could be down to air intake or lambda sensor playing up

I was driving home today from work doing about 35/40mph coming up to a junction and the car cut out, - obvs lost PAS. Any ideas what would cause this?? Ive looked online a few forums have pointed towards fuel pump?

Thanks