Bell internet keeps shutting down?

[Awesomedude1]

So i have bell, about a two year old modem, but the internet is typically really good in terms of ping and connection. like 22ms o-o, anyway, occasionally the internet just stops working, and 100% of the time, whenever anyone in the house, upstairs or downstairs downloads something, or is uploading a large video (past like 30 minutes) the internet just stops working until the download or upload is done. Any idea why? is it because of the old modem? What is the problem here?
this happens to every computer in the household and is more frequent in the night (i live in toronto)