I had the same problem recently
in my case it appeared to be a dns resolve problem.
I'm not sure if it's your problem as well, try to do something like this command:
the lookup should be done in about a second (containing a few ip's belonging to google)nslookup google.com
does it work properly?
if it doesn't you can specify alternate dns servers in the web-interface: ip config -> wan & lan > WAN DNS Setting
I chose for the opendns servers(free to use), and transmission seems to work fine now