Touareg :: Engine Turns Over / Keeps Cranking And Finally Starts With Slow Idle

My car is a 2005 V6 Touareg. It has 60.000 kilometers on it and had no major issues to this date. It was well maintained. Recently, it developed a problem with starting. Before I go into more details, I have to tell this: my car has two batteries, and they are in perfect condition, verified by the dealer. The spark plugs were changed at 45.000 kilometers. It has no rough running characteristics under heavy acceleration or any hesitation - from what I read, symptoms for bad ignition coils-.

When I turn the key for ignition, the engine turns over but instead of immediately starting, keeps cranking for 2-3 seconds, and finally starts with a slow idle. Sometimes the charging light remains on for several seconds due to the difficult starting and the slow idle, but this does not last and the cars start idling normally, as if nothing happened. This happens when the car sits overnight and the engine is cold. Once it starts, I cannot get the car to do this without waiting overnight. Summertime weather seems to contribute to this but I still get it in the cold winter weather.

Strangely, if I do not use the car for, say, 3 days, the car starts like the day I bought it, without the hesitation.
I took it to a dealer and told them to check it but they said it start normally. They checked to see if the car registers any error but it does not. I am sure of the quality of the fuel I am using. In any case, I tried several different brands from different stations just in case, but this did not make any difference.

Bad ignition coil, idling sensors or mechanism, something wrong with the intake manifold? Carbon build-up (although I have to say this is not likely since the catalytic converter seems to be fine which leads me to believe there is no excessive carbon buildup).