This is exactly my experience with Premiere CS6 and CC h264 export.
The artefacts I had are not as bad as you show in your pics, but always exist in every export.
Now, I'm using UT-codec to export and convert it without artefacts with TMPGenc x264.
I reset bios settings and all becomes OK.
Did you overclock your system?
Yes. Not me but computer master.
And he said that overclocked - OK, temperature is good.