Not trying to insult your intelligence, but is this a new fault or are you expecting too much from the car. It is a big car and on 17" wont be amazing in the wet.
It's a fair question
When I bought the car in 2007 it came with worn out asian strange named tires and the car seemed like it was glued to the ground
I then exchanged them to Goodyear Excellence which I found to be a bit too soft for the car, so after they got worn out in 2010 I got the ones I have now... Continental ContiSportContact 3.
After I got these tires the car seemed unstable to me i.e. drifting out from curves in the wet... and on the highway around 120Kmh (roughly 90MpH) if it is windy and wet then I feel the car drifting.
Withing these circunstances I would expect the VDC to try to keep the car on track and to see the VDC sign on the MDF, which doesn't happen
Before the Alfa I owned a Rover 414 Saloon (family things...
) and on wet surface it performed better then the Alfa 159 as it is now, which doesn't make sense since the Alfa has a lower center of gravity, is heavier and has larger tires...
And just for the record... before the Rover I owned a Lancia HF Turbo and before that an Alfa Romeo Sprint Veloce 1.5.
So keeping it short (which it isn't by now
) I don't believe that I'm pushing it over the edge or expecting too much from the 159