Hello Forum.
I am new on this site so please be gentle 
I have an S6 from Oct. 2006 with 233.000 km. I have had it for 2 years now.
Idle at cold starts are rough for the first 60-90 seconds.
When 10 deg. outside temperature it starts OK with rough idle for the first 60-90 seconds.
When 0 deg. outside temperature it is very rough idle, “almost misfires” for the first 60-90 seconds.
When -5 deg. outside temperature it misfires on randomly cylinders for the first 60-90 seconds. If I give it a little throttle and hold it at 18-2000 rpm there is no misfire but rough high idle.
As soon as the 60-90 seconds are gone and the idle drops to the normal 640ish rpm the car runs smooth.
If I use the parking heater and heat the car before start, the idle is smooth, even at freezing temperatures.
If I understand it correct, the first 60-90 seconds are some kind of “catalytic converters fast heat up programme” ?
VCDS shows no errors, except for random misfires at freezing temperatures if I do not hold my foot light on the throttle.
Recently I had an injector that failed so all 10 injectors are now new, also the sparkplugs and coils are new. Same as fuel and air filter.
Shortly after I got the car, the engine got a renovated block, as cylinder 5 and 10 was scored and made metallic knocking noise. I knew about this problem before purchasing the car.
Idle at cold was rough before and also after engine renovating.
A year ago (20.000 km) a new intake manifold was installed and also a carbon cleaning was done.
The rough idle at cold is not new, it has done that since the day I got the car. What new is that for the first time since I got it, there has been temperatures here in Denmark below -5 deg.
After the 60-90 seconds when cold, or when hot, it runs smooth.
Long Term Fueltrim reads +/- 1 % on all four banks.
Any idea where to look.