Admin pages often time out behind proxy

I’m currently designing a school website in England that’s not gone live yet. I’m waiting for 3.0 before I launch properly however I have a strange problem.

Whenever I’m in school and working behind our proxy the admin back-end pages often time out. When I refresh the page the content appears in the correct layout however the rest of the CSS is not applied and pages show with the default blue link styling and nothing else.

When I’m at home everything works fine.

The only thing I can think of is a DNS issue (as the pages showed this way for about 12 hours while the domain was pointed at our host) but its intermittent so I’m not sure.

Any ideas on how I can go about sorting this?