Private file downloads get truncated

I changed the file access from Public to Private in my Drupal installation, and now certain images get truncated when I try to download them. They didn't get truncated when I downloaded them publicly.

They don't all get truncated at the same number of bytes either. For example:

• DSCN5553.preview.jpg ( bytes), when accessed via ?q=system/files/images/DSCN5553.preview.jpg, the browser only returns the first bytes.
• DSCN5555.preview.jpg ( bytes), when accessed via ?q=system/files/images/DSCN5555.preview.jpg, the browser only returns the first bytes.

Does anyone have a clue as to what might be going on here?

The code for file_transfer() looks very simple and straightforward; I don't really know how I would debug it, though, as there is no opportunity to write anything to an HTML page (since it's a binary file that's being downloaded).

Any help would be much appreciated!