Pretty easy solution tbh.
Are you behind a router? I will assume yes.
Go into your router, go to port triggering, and add in Steam’s ports.
This will open those ports for Steam when you use it.
Those ports are:
Steam Client
UDP 1200 (For the Friends service)
UDP 27000 to 27015 inclusive
TCP 27020 to 27050 inclusive
http://support.steampowered.com/cgi-bin/st…_faqid=11#ports
See if that helps. Oh, btw. When you add those ports in, reset your router and cable/dsl modem just so you get a nice fresh shiny new connection.
