I bet it's a brown-out of some sort which will be fixed by bumping up the voltage a bit. Or using the Intel trick of slowing the CPU down when it needs to execute heavy vectorized workloads. I wonder why this only seems to happen under Linux though.
In my experience on older machines, it's possible why it's happening on Unix based OS's and not Windows is because Unix based tend to get more out of your hardware than Windows and this sort of hardware instability shows more frequently.
Could be. Linux has a very efficient mutex implementation, and as a result concurrent programs tend to spent a lot less time idling, and thus potentially load the cores more.