Maybe your problem’s been solved, but if not, similar ones have been covered quite a lot before in these forums, so did you try a search?
Sounds to me like a software problem, in which case you could check it out yourself (wherever you are), and quite likely fix it. Maybe you’ve already scanned for malware, but it might be legitimate software getting out of hand.
Your posting rang a bell, as I’ve been there too, fairly recently, with identical symptoms, and randomness, and cumulative effect. It affected my capable desktop, as well as my laptop. In my case the main culprit (by a long chalk) turned out to be an additional (and superfluous) security program my bank’s website was pushing.
You’ve given plenty of helpful info, but if you need to post again, your Windows version would be good addition!