1st January 2017, 4:49
It recurred twice more since the preceding post, + a few days ago Tower completely froze [even REISUB was ignored, necessitating a hard reset]. That was the last straw, so yesterday i blew away all vestiges of Mint 17.x KDE4 that might have been fighting with Maui [manifesting in a raft of annoying hassles caused (i suspect] by my original decision months ago to convert my Tower from Mint 17 KDE4 to Maui but reusing its encrypted /home partition]. Hence yesterday i rebuilt Tower with a complete SSD format & clean-install of Maui. i'll now mark this thread provisionally Solved.