capturing "server too busy" error message

ive finished making a site: www.loveisblind.org.uk and it's getting a good few hits at the moment as the charity are promoting it... problem is - it's also falling over when the server is too busy... in firefox, I get the message

The site could be temporarily unavailable or too busy. Try again in a few moments.

Is there a way in PHP maybe with .htaccess to capture this error and display a user friendly message? There are no problems with bandwidth as it's monitored by myself and they have a very high limit which they're no way near....

thanks for any help! :)

If it is an official Server Error message then you can create Custom Error pages
http://www.w3schools.com/tags/ref_httpmessages.asp