you're running 3B+ at 1.4 GHz, do you have sufficient cooling for it? I've a 3B+ too and when viewing the video stream, it would overheat at stock speeds. I had to underclock/undervolt.
the printer using Marlin 1.1.0-RC8, which is ancient (release date: Dec 23, 2016), you should upgrade to at least latest of Marlin v1 (1.1.9)