This is more of an Internet Explorer / OS 9 issue than an iMac issue, so don't be surprised if the topic moves.
One thing I would try is to use a different web browser if you can (such as Netscape 7, which is available
here). If the problem still happens, you know it's an issue with OS 9 itself. If not, then something's wrong with IE.
I wouldn't be able to help in earnest if the problem happens with Netscape or another non-IE browser as well, but if it's just IE then you can try deleting IE's preferences (they should be in the System Folder) and maybe reinstalling IE itself. You can get the classic Mac OS version
here.