CPU speed goes down to 0.2GHz when charger is connected.

[Deuteron]

My computer is 4 months old. For the first 3 months it was fine. Then suddenly it started doing this. (presumably after an automatic update) When running on battery, it works fine. When a charger is plugged in, the CPU speeds up to about 3.6GHz for one second and then it goes right down to 0.2GHz and stays there, The CPU is cool, about 44 Celsius as seen using the free CoreTemp utility. I cannot work with a CPU that is operating at 7% of its normal speed. After browsing about I found that if you install "ThrottleStop" and uncheck "BD PROCHOK", the problem goes away. However Throttlestop is NOT an acceptable solution; Turning overheat protection could be really bad for my computer. An independent HP service provider (CMC El-Masudi St Jerusalem)  told me that this is a Microsoft issue not an HP issue, so they could not help me unless they gave a service for payment, and I would have to bring the computer to them. 

Suggestions are welcome.

[Deuteron · Accepted solution]

A service agent called Omer at HP service in Israel provided the solution. Updating BIOS to V14 fixed the problem,  Prior to this the computer did its own automatic BIOS update. It is unclear what triggered the automatic BIOS update, and it is unknown whether the automatic BIOS update caused the problem, or whether it was a different automated update.

To perform a BIOS update yourself, go to https://support.hp.com/us-en/drivers/laptops and follow the instructions, That is what I did as instructed by Omer at HP

Note that the advice provided by the CMC, the independent service provider, was wrong; this was not a Microsoft Issue. Thereafter, HP provided a good solution