Hi Dario,
I do have some ideas but rather than just throwing random techno-babble at you that won't help you fix this issue I'll give you some advice that will
You might find this query better put on
http://www.dslreports.com/forum/comcast where other Comcast customers and the odd staffer can advise, and indeed you can get help from Comcast reps directly in the Comcast Direct section.
This forum is quite strongly UK centric, there you're in with a pretty good shot of having a Comcast staffer picking up the case.
Thanks!