Is there a cure for slow DNS lookups?

I'm having significant delays with my browsers (Firefox and Chrome) looking up Internet site addresses. Lookups often take 5 seconds or more, which is a lot when there are often 10-20 lookups required to assemble a single Web page (mostly to add advertisements...)  I also have pretty frequent lookup failures (the browser reports that site can't be found -- even sites like Google and Yahoo.)

I have my Internet connection set to automatically select DNS servers (per advice from Cox.)

Is anyone else experiencing this? Any good ideas to speed things up?

Thanks!