Freddy: Intermittent Access Denied SBS2003

Please offer up any troubleshooting hints for this: I have one XP user who started getting intermittent access denied errors when opening files from two folders on our SBS2003 server. It changes hourly, on the same file in the same folder. I've struggled with this for days now, and am concerned it could spread to other users.

I've tried a number of solutions, none of which provided a lasting fix, including

1. Deleted the user from the folder's ACL. Created a new user and added that to the folder's ACL.
2. Logged onto another PC as both the old and new user. Same problem.
3. Copied the folder to the SBS2003 desktop, renamed it, copied it into the same directory.
4. Removed the user from any security groups besides domain user.
5. Logging out and logging in multiple times.

The hard part is that I keep thinking I've fixed it, but the problem keeps coming back, like Freddy.