Strange DNS error.....

Hi, I am getting a strange DNS error - this does not happen all the time - Every now and again when I am browsing the net I get the message "Cannot find server or DNS Error" when I know for a fact that the website I am trying to reach does exist. Check my /var/log/mail log file I am also seeing that when sending out emails, my MTA (sendmail) is rejecting a portion of emails with the message "host name lookup failure" - again I know for sure the hosts it is trying to connect to exist. I don't use an ISP so I am keen to try to work out why this is happenning - anyone know why how I can track down why this is happenning ? Sorry if this is a bit off the subject of Linux... Dan