[support] Problem with file downloads

Hello, I have users uploading files using IMCE. For some reason, *some* *times* when you try to download a file by visiting: http://mywebsite.org/files/userxx/some-file.zip Drupal gives a "page not found" error. I don't know why it does that. I've looked all over the admin pages and I can't find anything that looks relevant. I thought that Drupal had a setting to have Drupal manage file downloads instead of doing directly from Apache, but for the life of me I cannot find this anywhere. Help? Daniel.