Hi,
I'll respond in English since it is much easier and you do understand it obviously. I am also a CableVision subscriber in Brooklyn an lately experienced the same exact problem at night. My connection speed was as low as 1-2Mbps. After call with Cablevision and discussion with tech I've comfirmed the fact that our street block used only one shared connection node and every single user on the street was sharing connection. I told Cablevision that I am not willing to pay for such level of service since I only use computer mostly at night and they offered me a credit until they could resolve the situation. It took them almost a month but after that my connection is steady at 20Mbps throughout the day (Cablevision advertised rate is 15 max).
I am about to switch to Fios since they offer 25/25Mbs connection and it is guaranteed since you are not sharing it. It suppose to be much better than Cablevision's service so if you have an opportunity to switch I would do it right away. Currently they offer tripple play promotion for $84.99 where you can also get multi-room DVR for free for life.
Best wishes.