Page and Browser Caching Breaks Large Encrypted link downloads

After installing w3 total cache, several larger download files return Error 6 (net::ERR_FILE_NOT_FOUND): The file or directory could not be found. when the encrypted like is returned from another plugin. I would like to resolve this. It works fine on files smaller than 1m but on several larger files (9-13m) the file not found error is produced as soon as I enable caching of pages or browser caching. The only caching currently configured is cloudflare (no object or DB caching is enabled either). The conflicting plugin is wpStore but I have seen folks with Download Protect having the same size based problem. (See: http://wordpress.org/support/topic/plugin-download-protect-dp-stopped-working-on-some-files-still-works-with-others.) I suspect there is a size setting somewhere that would help fix this so that I could keep my encrypted links for purchased downloads but I have not found it. In my case they are also pdfs and I was not able to find a way to exclude them either (at least not without commenting out the line in the .htaccess–which I was loath to try). Any help would be appreciated.

http://wordpress.org/extend/plugins/w3-total-cache/