503 errors mean you are loading too many pages at the same time...
The site prioritizes logged-in users. If you browse the site when you're not logged in, you're much more likely to experience 50x errors.
I use Tor.
Once I am logged in
Hello Rollyco,503 errors mean you are loading too many pages at the same time. There is a hard-coded limit of 5 concurrent connections. You are more likely to see this when the site is busy, because pages take longer to load.
FYI, logging in to an unencrypted website such as this one through Tor is very insecure. Your password and 100% of your traffic are easily sniffed by any exit node operator willing to spend 10 minutes setting up some software.