Using Imac and Macbook. finding websites

Using Imac and Macbook. Recentlly I have been having real problems going to websites. - I can google but when I go to the website I'm getting "safari can't find the server" - the same happens in firefox. I switch between the two and sometime this works. This happens every day and it is so intermittent that I can't work out a solution. I have contact my interney provider and I am not experiencing any outages.