Since last week i was experiencing random access denied messages on our sharepoint 2007 portal. the problem seems to be not appearing in any of our event log and sharepoint log. After some brainstorming and debugging i found out one site from microsoft.

http://support.microsoft.com/kb/953459

It says in your web.config, batchupdate is not supported at root level of your site config. If you have accidently opened it to have custom errors then you need to make it false. Else it will start behaving abnormally.

So make changes in your root web.config to: <compilation batch="false" debug="true">

Hope this helps !!!