>>> the fibre-optics router power is low at the moment - could this be a cause of it?
Not as far as I can see.
The problem you describe is usually associated with something hogging all of your PC's processing power or memory. The fact that it occurs periodically suggests that it could be an update process (e.g. for Windows or for your security software) that's taking place. All I can suggest at the moment though is to keep the Task Manager running, but minimised, and when you encounter a problem try to bring it up to full screen (which obviously won't be that easy with a sticky cursor, I know) in order to identify what it is that's causing the problem. Then, if the answer isn't obvious to you directly, we might be able to advise you as to the best way forward.
(If you're unfamiliar with using Task Manager, please refer to my post here:
https://www.theanswerbank.co.uk/Technology/Question1714769.html ).
It might also be worth running a malware scan. Download and run AdwCleaner. (There's no installation process involved. You simply run the downloaded file directly):
https://www.malwarebytes.com/adwcleaner/