Found that the ‘latest’ (11.2.x) Adobe Flash Plugin for Linux (x86_64) makes YouTube videos look like something from a bad trip.. Tried different browsers, but the fault was still there.. Videos from news sites (BBC etc.) seemed to be OK..
So – reverted to 11.1 version of Flash Plugin (from Adobe archives) and all is well, again..
As the 11.2 version is supposed to be the ‘last Flash for Linux’, I hope they fix this soon!