It could really be a large number of things.
-Spyware or Malware could be blamed for bandwidth waste.
-Sure you don't have any torrents runing, or a filesharing program like Limewire?
-Alot of Source players use Xfire
. Alot don't know that Xfire automaticly downloads updates and patches for their games.
-I remeber playing ole' Source alot, great times! But I had a DSL modem that would get incredible warm and would actually shoot my ping on a 100-tick server from 60 to several hundred ping. And since you said it takes a while it could very well be the problem. I actually put a ice pack on it acter restarting it and it worked like a charm, but be very careful about condensation and water leaking it.
That's all I can think of off the top of my head. If you think it's the malware or spyware you may want to start Here
Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning.