I suspect this may have something to do with how much a shared webhost overloads its servers with websites. When I was at a host that crammed its servers as full as possible, I had choppy FTP problems. After moving to a host with fewer websites per server, I've had no problems.

When you log into FTP, you get a message that says something like "you are connection number 2 of 50 allowed". Maybe when the remote server's FTP program reaches its connection limit, it automatically terminates the oldest existing connection to avoid having to refuse connection to somebody requesting a new connection?

Some workarounds I can think of:

1) If you can do file transfers within cPanel's File Manager, maybe you can get better performance and connection stability there than with FTP.

2) Manually create the folder structure you need on your remote server, and then FTP the 20MB worth of files a few at a time so you don't need to use a long FTP session to transfer the whole 20MB at once.

3) If the 20MB file really is a single file, use a program to cut it into smaller pieces, transfer the pieces, and then reassemble them on the server. Linux utilities called "split" and "cat" can do this. If your local PC is Windows, I'm not sure how you'd do the split locally, though. This workaround may be more than you, or most people, would be willing to attempt.