
Yup, it mostly happens when torrenting - and the problem is mainly somewhere in the DNS server. Your IP is leased for somewhere around 5 hours, and when those 5 hours are up, if you're torrenting the DNS server doesn't give you a new IP.
A small fix for this is to type in "Start > Run" the command "ipconfig /renew" before you start experiencing problems, and remember to do that every 4 hours or so - won't help if you run it after your connection is down.
Also, unplugging/pluggin in the modem will always do the trick when the connection fails because of this torrenting stuff.