The BIOS had corrupted the Q-fan profiles and turned one chassis fan off entirely, put the others in a mode where they were just barely running so the machine was overheated. A BIOS upgraded fixed the fan issues.
While I was there and letting a torture test run for a while (8 copies of an AVX version of prime95 running small fft’s, about the most torturous thing you can throw at a modern CPU, I worked on some other issues and discovered the problem with postfix not starting on debian, missing postfix and postdrop groups and postfix user. I’ve fixed this problem before but it turns out the post.install script of the postfix package distributed with stretch (and Ubuntu 16.04.3 LTS) is broken. So it will likely break again the next time a postfix update is kicked out.