Youtube video (or any video) stuttering and crackling audio - apparently linked to CPU throttling

[Cassini]

Hello,

In mid-May my laptop started to suffer from freezing/stuttering of video (Youtube, Facebook Messenger video calls etc). I determined that it was not my browser (Firefox) or my connection, by using other browsers and other devices. I've spent the last two months trying to chase down this problem. What I can say is that I have tried all the usual stuff and then some - just some include the 'sfc scannow' command to detect bad files, driver updates, a new BIOS/UEFI (Insyde F.91), various DISM health check/health scan/ restore commands to try and detect bad files, plus changing the power plan in Device Manager (weirdly, I just this week discovered nearly all my power options have disappeared from Device Manager - another Microsoft update I fear). I am linking this trouble with what I found (after the fact) was a Windows 10 update for build 1803. LatencyMon reports pretty bad DPC latency when the video problem is occurring - milliseconds. Nothing has worked.

However, I do have a smoking gun clue - after downloading a free app to check that the CPU temperature wasn't getting too high, I notice that the video freeze/audio crackle only occur when the CPU clock frequency automatically adjusts to 499Mhz (x5 frequency multiplier) and the core voltage to ~0.66V. Other times, the app reports the Core voltage is now ~0.87V and the core frequency 2495 MHz(x25).

I believe whatever controls the thermal status of the CPU is, with whatever algorithm it uses, often throttling my CPU to the point where it can't handle video or audio. Task manager can show 100% regularly during these times. No one process is really hogging the CPU, it looks like CPU % for everything is inflated - even task manager itself can take 10% of the CPU during these periods. Of course, maybe some processes are running inefficiently and ordinarily even with the CPU VID voltage and frequency throttled down they should run OK, but aren't after these May Windows updates (KB4552931/KB4556799).

Can anyone offer a similar experience or any clues? (NB: The CPU temperatures aren't anything extraordinary, it does get above 60C on occasion but I can't really link them to the CPU throttling anyway)

[Cassini]

Update: Using an app called Throttlestop I have determined the 'BD PROCHOT' (BiDirectional PROCessor HOT) register flag is set for my CPU and Graphics processor (although not for an item called 'RING'). This should only be set when the CPU temp exceeds 99C I believe. My CPU is reading 46C! This triggers my thermal control software to throttle the CPU right down.

Throttlestop allows me to disable this BD PROCHOT flag and when I do the processor frequency shoots up from 499MHz to about 2.4GHz. So, I believe I have a dodgy PROCHOT sensor. Not sure what to do about this exactly but I can use Throttlestop to blank it. May try a hardware reset where I unplug the power lead, battery and then hit the ON button repeatedly for 30secs to drain the power out of the system (assuming there's no backup battery for RAM - which there may be). Anyway, hope this might help someone in future get to the root of this sort of problem.