OK I disabled all non-MS shell extensions. There were 36. I will first use Sigil awhile to see if it seemed to fix the problem. Then will enable the first 18 and try Sigil again. If after a couple days it's still ok, I know the problem shell is in the second half... so will enable 9 of those, etc, working my way down to the one. Will report back in some days...
Thanks for this as I can just enable all of them again when I am not using Sigil. Then disable again when I am.
EDIT: In the meantime I am CTRL+S'ing my a-- off.