Technically the server has been up near forever, but your site may be an another matter altogether. I'd recommend that you sign up for a (free) third-party monitoring service, such as host-tracker.com. That way you can have more practical information to work with.
As far as DH reliability has been concerned... when we first signed up here at the very end of 2004, we got assigned to a server that was extremely flaky. We complained enough about it to support, and they eventually moved us to a new server. The new server has been behaving like a champ months on end, our sites clocking near-100% uptime save for scheduled maintenance periods. Only the database server gets s-l-o-w occasionally.
I'd suggest that you submit a support ticket, and describe exactly what kind of a site you have (database-driven? PHP? static?), how it goes poof on you, and most importantly, that your site has gone down repeatedly. You might also mention what the web- and database servers you're using, so they can save time if it's a known problem.