Audi A6 2.0tdi C7 2018 S tronic gearbox, 64k miles

Hi,
I have got this strange problem, the car stalls occasionally on a start or on traffic lights when about to move again.

The problem is coming randomly but 2 things happens before it does that
1. Car changes rpm from 900 to 1000 at idle
2. When driving at a low speed or in town for some days

I went Germany and drove at 80 to 90 miles fully loaded for 5 to 6 hrs on average so the problem went away.

But came back when I returned to town driving in the UK.

It judders as well, again occasionally.

Majority of the time it drives fine from cold or hot start but then it has happened on a cold start as well.

So there are no patterns as such for the problem.

But I have noticed the fuel average has gone down to 42 mpg motorway (driving between 70 to 85) and lot lower in town, where as other cars of the same specifications are giving around 48 to 52 on average on motorway.

Any help to put me in the direction to start the investigation would be helpful.

Thx in advance