404 Error Messages, login prompts and sluggish SharePoint 2010 performance.

Hello,

I have a SharePoint 2010 server and some users report random login prompts, but others do not get prompted.  They are using SharePoint for a little while, and then SharePoint prompts for credentials, or the prompts happens clicking to a new site.  Users have the sites trusted and have the automatic login with current user name and password option set in IE.   

Also, I have 2 other SharePoint 2010 servers in other offices with sites and when users click on theses sites, "404 Not Found" often messages appears.  This happens when users click quickly to these sites.  When you wait for 15 seconds or so before you click to go to the sites, then you won't get the message.  

Users often report SharePoint runs sluggish for a little while, and then runs good for a little while. Performance is up and down on the server.  There are 5 or less users on the site, so very little activity on the server, but still SharePoint will be sluggish.   Memory and CPU on the server are good. Often times when you are doing some saving, opening, and copying files within SharePoint an error will occur or the IE will show that the site is not responding, but other times operations like this are good with no problems. 

Does anybody have suggestions on what could be going on?

Thanks.

Paul