Right, sorry that didn't help.
Do you have any extensions enabled? Flash?
The best suggestion I have is to try Safari Technology preview
Safari is the best way to see the sites on iPhone, iPad, and Mac. Safari Technology Preview gives you an early look at upcoming web technologies in macOS and iOS.
developer.apple.com
It is a preview of what's to come in Safari and with a bit of luck, it might have a fix that solves your problem.
The only other suggestion I have is to enable the develop menu (found in preferences under 'advanced')
and try disabling things there.
It is an odd one though as it's clearly specific to something you have (and a few other people) and it isn't a general problem. Especially on a new install.
I suppose it could be malware? Very odd though.