Please attach a full session log file showing the problem (using the latest version of WinSCP).

I just downloaded Core FTP LE and I'm trying that now. The problem is not there. File transfer speeds vary, but are generally fast, and I'm not getting any errors.

I prefer the WinSCP interface but I'll stay with Core FTP LE until we can get the issue resolved in WinSCP.

Please let me know if there's any more info (settings, etc) I can provide to help troubleshoot!

Did you try other SFTP client?

Thanks for that question; I meant to add the detail. After the message pops up, the transfer will always resume after a period of several minutes, however the speed resumes at a painfully slow rate, (never restarts at ~6MB/s, more like 1MB/s or less) down to zero, then "Host is not communicating" pops up, and the cycle will continue.

My source machine is a PC running Windows 10, and the target is either a Raspberry Pi Zero-W or a Raspberry Pi 3 B+. The problem seems equal on either target machine now, and I previously never had the issue with either.

Thanks for your help!

After the "Host is not communicating" message pops up, does the transfer continue eventually, or does it stop altogether?

Recently my file transfer speed will start at a good rate (~6 MB/s), but then after several files transfer, the speed will reduce to less than 1 MB/s, then go to zero and I will get the "Host is not communicating ..." message. If I restart the session, same thing happens...fast connection & file transfer speed, then bogs down after 30 seconds or so. I've read all of the posts on slow transfer speeds & what to do to improve them, but this is different.
I've been using WinSCP for years to transfer thousands of music files in a session, with few issues, but this is new. No change to PC nor operating system (Windows 10), nor changes to router.
Any insight would be appreciated.