IIS FTP Giving 997 Error

When attempting to download a file through FTP on a Win2k3 server with IIS, the download fails the first time and then always succeeds the second time. The IIS log is showing error 997 at the point of failure which translates to “Overlapped I/O operation is in progress”. I can’t think of any process that could be accessing this file. IIS should be able to handle multiple I/O threads. Can anyone think of what is causing this? Any tools out there which could narrow this down?

Thanks in advance.