One core is always near 100%

I noticed this problem after making it dual boot with Linux Mint and Windows. Running either OS, one of the four cores is always running 96% or so. Which core is affected changes; it’s not always the same one. Can anyone tell me whether this is some sort of firmware or HW problem that can be fixed?
Also how do I get another copy of Windows for this, if I chose to wipe the drive?

BTW, the SSD on my other Herobox died, so I got a new one and an running Mint, which works great.