Intermittent "Access Denied" on a Document Library

I have 120 users in an Active Directory group. This AD group has Contribute rights on a Document Library in Microsoft Office Sharepoint Server 2007. When users upload documents to this library it starts a Nintex 2007 workflow. All users produce their document and access SharePoint through IE7 on a common Citrix environment which is hosted in the same rack as the SharePoint server.

There are 4 users who receive an intermittent "Access Denied" error when they try to upload documents. Sometimes it works, and sometimes it doesn't.

Has anyone observed this behaviour or know of possible causes?