CasperJS / PhantomJS hanging on casper.start on localhost when connected online

I have a simple CasperJS test asserting an http status of 200 on http://localhost:3000. It works as expected (fast) if I am NOT connected to the outside network, but if I turn on Wifi, it takes 1-3 minutes to return a test result. Connecting to http://localhost:3000 in my browser works fine regardless of internet connection.

The Casper logging/debugging features have not revealed any clues.

Any ideas what could be going on?