Weird problems with the T420 ThinkPad

[Sewan]

Hello, I've recently started using my T420 again but I've started to get problems I've never experienced.
CPU: i7 2620M
RAM: 8GB 4GBx2
iGPU: HD 3000
dGPU: NVS 4200M
SSD: Crucial BX500 256GB


The first problem that I have solved after doing some research where my CPU would run at 810MHz. I had a 65W charger and a battery that is currently at 37% health. after getting an oem 90W charger that problem went away.

The second problem that I can't figure out is that one of the CPU cores is not being utilized in Windows at all. Basically in Windows when running any program or even at the desktop I would see that in task manager the first core with its thread was being used but the second core wasn't being used but the thread of it was. That basically meant that the CPU was running at about 86% usage under full load (CB R23). This problem only occurs on Windows 10 and 11 (I haven't tested 8.1). Where it doesn't occur at all is Linux or Windows 7 with the original restore DVD. I noticed this before I replaced the charger but even after replacing it that didn't go away. I also tried resetting the BIOS to it's defaults, and flashing the BIOS to the latest version with no success. Does anyone know what the cause of this could be? I've looked thru the internet but I didn't find anything about this weird issue.