firewall/router settings. make sure your firewall is allowing steam/css to access internet. Make sure your router isn't preventing connections to/from steam/css and CSS ports in general.
pick a CSS server, get their IP address. go to run>cmd> ping the IP you got to see packet loss and ping. If nothing wrong here, do a tracert to the same IP, if you cannot finish the trace that is a problem.
Go to NIC.com and see where the IP is registered to, then go to pingtest.net and test your connection to closest server to the IP you looked up.
It's possible your ISP managed to screw something up. One time I had a lag free experience with comcast, then i went on vacation for 2 weeks, came back, connection was unstable and laggy. Spent 2-3 months arguing with comcast, end result, switched to another provider.