Can't view only one website, my own

For the last 3 days I cannot get into my website at my one location. All 4 computers cannot get to the website. When I reboot my modem, I am able to get into the website, but for only a short time. I made sure that the website is not blocked and even added it to my trusted sites but to no avail. I tried pinging the site and even changing my broadband DNS. The problem still persists. I contacted my Internet provider AT&T (Bellsouth) and they cannot fix the problem. Help!
Mark

flush all cookies, quit browser, then

get a command prompt (from an ADMIN login) and try
ipconfig /flushdns
net stop "dns client"
net start "dns client"

now use your browser to access the site

Thank you. I will try what you suggested and then let you know the results.
Mark

Hi, I tried what you said and this still does not resolve the problem. I am beginning to believe that there is a DNS poisoning in my AT&T 2Wire Modem/Router.