IMCE causes an incomplete file download

Using Drupal 6.13, IMCE 6.x-1.3, IMCE MkDir stable, pure SSL (no mixed sessions), and private file system.

Recently got a report from the client that after downloading a file, she noticed that the file (while readable) was incomplete. This was a dataset with .dat extension. Client was using latest (3.6ish, or at least recent) Firefox (!) on Windows (not sure what version). She was logged in as a non-admin user. She said no error occurred during or after download, but that the file size of the downloaded file did not match the file size listed in IMCE. This leads me to believe that the file transfer resulted in an incomplete download.

Is this an IMCE issue, a D6 private file system issue, or an issue due to SSL and host (Rackspace Cloud Sites)? Another possibly related issue is that NO files were downloading in IE7/8 until I hacked the cache control header in the core file_download() function. Also, after implemented pure SSL, there was an issue where the first file that a user tried to download after logging in resulted in an error and corrupted download (only in IE). However, since this current issue occurred in FF, I am reluctant to blame the HTTP headers that Drupal is setting and suspect some issue in IMCE file handling.

Has anyone else experienced this issue or can offer advice about faulty file downloads through IMCE? I cannot provide a link since the site is in production and the data is sensitive.

Thanks for any help floating around out there. Client views broken datasets as a dealbreaker and I'm really hoping to avoid WebFM.

Dakota Ryan