I have started seeing this issue again quite frequently in the past few weeks. My CPU heat sink and fan are clean and working just fine.
It happens when watching YouTube videos. Look at the CPU and power usage in Task Manager. I typically have only a few tabs open in two windows. I use only two extensions: WordWeb and the Vivaldi Forum mod.
YouTube CPU Power Usage.png
I noticed that the videos are 1080p HS. Reducing the quality to 720p reduces the power usage to High/Moderate.