There is an extra element here... All of what you said is true, but the actual issue is not with the video decoding, but with the thread that feeds data from the network to the video decoder. That thread has some long running stuff running on it, which causes the task to enquire data to the video decode hardware to be delayed.

It used to work years ago, but as more and more software bloat has been added to run in this thread, the delays have increased to the point of stutteryness.

Some videos are jumpy because they have fewer seconds buffered in the hardware decoder.

Pretty much every single processor made in the past decade, if not two, has a core temperature sensor along with overheating regulation. Even the Raspberry Pi.

Can't YouTube serve up the videos in formats that the original Chromecasts can consume? Surely either YouTube can know which Chromecast is making the request, and serve up an appropriate quality, or allow the Chromecast to request a lower quality version of the video. To be clear, I don't expect to be able to see videos in higher quality than was available at the time of purchase. I expect to be able to 'watch YouTube' as was advertised on the device's box.