sharepoint intermittent access denied error

My users keep getting intermittent access denied errors.  I can't reproduce them and no one is doing anything strange.  They are just browsing our site normally, one minute it works, the next they get access denied, then after about 30 seconds it works again.

Everyone is getting really worried about sharepoint though.  This is a very big problem for me.

I could really use some help here.

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Greg BurnsConnect With a Mentor SQL / SharePoint ConsultantCommented:
Couple of things to consider.  Since you can't reproduce it you should probably turn on verbose logging in SharePoint central administration.  

Are we talking about the "Access Denied" web page that SharePoint displays when you try to access something you don't have permission to see?  The one that has a link to sign in as a different user?  Or are your users simply being asked to log in again?  That would make a difference.

Checking the IIS logs probably isn't going to be much help because the "Access Denied" error is coming from SharePoint: it knows who you are and is refusing you access.  This actually helps isolate the problem.  It means yes, you're authenticated, but no, you're not authorized.  If you were getting an intermittent login prompt, that would be different.

Examine the application logs for .NET errors (particularly ASP.NET 2.0.50727 "1309" event).  Sometimes these "unhandled exceptions" show up and provide a lot of useful information.  But that's just one thing to try.

There are still quite a few things that could be at play here.  We really need to know more about your operating environment before we can be very helpful. What kind of domain are you running in?  Is this in a DMZ?  Are you using SSO?  

Instruct your users to email you immediately when they encounter this issue, so you'll have a timestamp to look for when you are examining logs under the 12 Hive.  

Feel free to paste any mysterious log entries in this thread.  Good hunting!
You might want to consider adding the webserver zone....

Have you looked at the web server error logs?
How many web front ends do you have in your farm ?

If you have more than one do you access the site using a load balanced url ?

If you do, then access the site dirrectly from each of the web front ends using http://servername and check if you run into the issue.

Greg BurnsSQL / SharePoint ConsultantCommented:

Is this site using Windows security or Forms based authentication?  Is it MOSS or WSS?
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