Issue: Site Unavailable errors

I don't imagine that anything can be done about this, but I wanted to let you guys know. Ever since the upgrade to vB4, it seems that I get the "site unavailable" or capacity problem error at least 2-3 times a day that persists for anywhere from 5 min to half an hour at a time. (The 50something error--apologies, I don't remember the last number).