All previous symptoms are abated; Free Mem remains clearly a ƒ(opened pages) and is stable.
High %CPU-usage as result of internal thrashing-overhead is gone, after months of s l o w
%Idle is 97-100% (when er, not processing a new task, of course.)

Nice. This iMac still rates its epaulets and DSO-with-crossed-palm-fronds.
(Even the earlier, very-occasional video-bars phenom has not recurred since I increased fan-speeds; it almost Had to be temp-related, I thought. Still do.

The GPU and (separate) GPU Diode sensors + iStat== exact relevant info.
iStat: remains (to me) best add-on Real Info evah! (even for newbies, if they are coached re using SMCfancontrol and Why.)
iStat==don't boot a Mac Without It.