Timeouts in Varnish
Varnish has a number of timeouts, most of which don't make it into the default varnish.vcl build by the Plone buildout recipe. This means you end up with the defaults. The 503 is generally caused by the first_byte_timeout. This is set to 60s by default.
To change it, just add the following to your varnish.vcl in
backend backend_0 { .host = "127.0.0.1"; .port = "xxxx"; .first_byte_timeout = 300s; }
There are other timeouts, and you can find more detail here:http://vincentfretin.ecreall.com/articles/varnish-guru-meditation-on-timeout along with some other useful links.