"One of my ISP techs suggested that it might be a MIME type issue -
but I have similar files for free download in another DIR with Phocadownload
with no issues."
This is possible. The Nice PayPal Downloads tries to determine what the file type is and then pass that information to the browser. It is supposed to fail gracefully, meaning that if it cannot, then it tells the browser just to do the download without knowing the file type. If there was an error during this identification process, it could hang the download. Plus, different servers have different file extensions that they recognize. If Phocadownloads does not try to identify the file type first, then there may be no error and the file would download no problem.
Let me take a look at a few things, then I will show you how to bypass the code responsible for identifying the mime type.