As Chris has already said, when the site is slow or appears down - please do not keep hitting refresh or trying to click links to other parts of the site.
All pages are served through a central database and when the site appears "down" its because this database is overloaded slightly. when you make more requests (click links or hit F5/refresh), all you are doing is adding to the queue of pages it is trying to serve, and eventually it gets into a right mess and cant recover due to the queue growing quicker than it can serve it.
We know the fix, and just need to implement it. As it requires low level database changes we are being very cautious about this and it requires Chris to perform it as he has the right access. Even after the change there may still be some tuning required so it might take a while to get things back to how they should be.
Unfortunately this is a our own doing becoming so successful. We are far larger than we have ever been and even in just the last 12 months we have twice as many visitors as before (actual visitors not bots), and also a lot more interest from places like Google who like to keep up to date with the site which is very busy. This means the DB is having to work harder than before and needs to be tuned as such, which it isnt out of the box.