The outage error has happened to me once in the past couple of weeks but it was quite short-lived.
This could be a sign that we will soon need to move the site to a dedicated server. At least, that's probably what the hosting company will advise. ( It means more $$$ for them.
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Last time something similar was occurring, I think it was caused by too many bots trawling the site. I noticed a marked increase in "guests" viewing pages and doing other stuff. The problem resolved once their numbers went back to normal.
A software update might also help, but that would take a lot of time. I have hacks in place which would need to be recoded, such as the extra questions added to the registration form. A job for next summer, perhaps, when I have a bit more free time.