Thank you for the update.
I have made a few additional changes can you please test this update and let me know if it is better or worse than the previous?
https://oss.azurewebsites.net/testr/dev-builds/ffxp4.2.5.1807.zip
I tweaked my original changes using Sun_SSH_1.2 running on OpenSolaris x86 under VirtualBox and saw increased transfer speeds and improved reliability. I hope you see similar results.