I'm assuming you can access other protocols fine, such as http (in order to have posted this here), mail, instant messaging etc, you also said 'most' servers. Maybe your router is blocking a certain port?
Do you actually receive an "* Unable to connect to server (Connection timed out)" error message or does the connection simply stop receiving traffic (which may be an MTU issue on your router)?
Without a more specific error message it's a bit of a stab in the dark, sorry.