Page not found, but not consistently

Hello!

(I apologize if a solution is already floating around out there; I really have looked for and tried a number of answers, but so far nothing has worked -- thanks for your patience)

I've installed Drupal (on my DreamHost account), and am getting "page not found" issues on some of the pages -- I know that there is already documentation related to "page not found," but I've already tried everything I've found, and thus far nothing has worked. More interestingly, the error does not happen every time; I might be able to hit the site, login, and do a number of things before I see the error. Similarly, I can refresh the error page a number of times, and then at some point (might be the seventh refresh -- it often takes a while) the desired page will come up.

This seems to happen whether or not clean URLs is enabled. I've disabled DH's "extra security" option. I have also verified that the appropriate copy of .htaccess is being read. Please help!

Thanks,
Dieter