How sure are you that the DNS infrastructure for your domains is in good health? Are they all properly delegated to the right place, with correct NS records and glue? Are all of the domains that you rely on to get your names resolved properly configured themselves?
Although everyone out there is probably saying "pfft, of course!", don't be so sure. Before you assume the best, try running one or two of your domains through Squishywishywoo's DNS checker, which does a recursive query for the given record from the root servers all the way down, giving you complete information on what's being checked and what the results are. From that report, you can easily work out what's wrong with your DNS lookups, and correct the problem at it's source. It's also handy for seeing exactly how a DNS lookup works (and explaining to random nitwits how DNS queries actually work).
It's actually interesting to run a few domains (run by people who should know better) through the checker and see what actually happens. There's a surprisingly large amount of breakage in people's DNS setups. So the next time you find someone's website is slow to load, perhaps run their FQDN through this DNS checker and see if it's their server or their DNS lookups that is slowing things down.