Many thanks for your excellent suggestions about my problem - sorry I didn't reply sooner; I've not been back to the forum for a while. Got frustrated and ended up using IE9 (never used it before!) with Task Manager manually setting the program priority to High and running nothing else to force the thing to perform without stuttering.
Firefox mis-rendered, and Vivaldi had too many modules to up the priority to stop it stuttering. Never heard of Pale Moon before - must take a squint!
Again, many thanks and I will now see if I can get Opera (which I've used since version 3 & which never stutters when running video streams) to work again - IE9 takes forever to load ....