"404 Not Found" message on SharePoint site when quickly clicking on link

Hello,

I have 2 SharePoint 2010 servers which none are in a farm.  If I am on one site and then click quickly to the other site, I get a 404 Not Found message on the site.  If I wait 10 to 15 seconds, then the page comes up.  When the 404 message does appear on the page, the error shows up in the IIS log, which say the page is not found.  But when you  wait 10-15 seconds, no error show in the log. The link is valid, and the page and the site exists.  Can you explain why the 404 error comes up with clicking quickly to the site?

Thanks.

Paul