Useless comments aside, I too have been having issues like this on other source games, particularly TF2, Counter-strike: S, and Garry's Mod.
I have been able to remedy this situation by creating a game and then connecting to a server from there. (current map or etc does not matter, just make the server!)
This is a treatment, not a diagnosis. I don't know what causes this either, but hopefully the above can get you out of this bind as it helped for me.