An update ...

Spent all day trying to get the board back to normal.  No dice.  One of the ASIC chips is fried (only reports 7 dies on the advanced stats page) and the temp sensor is mangled along with it.  I still get swings from +300C do -300C.  But after many reboots and other attempts, it's mostly reading negative low temps right now.  So it gets some hashing in.  Currently:

Core Temp 1 (°C)   65.22   48.64   54.80
Core Temp 2 (°C)   46.94   -216.28   -22.62

I have a hunch that -216.28 actually means 21C (the temp on the dead chip) and that 310C means 31C.  At someplace below 28C it reports a negative value (times -10) and somewhere above 30C it reports as a positive value (times 10) ... that's my guess based on what I've been seeing.  Those scalar multiples may not be exact of course.

So I'm trying to keep the board as cool as possible for now in the hopes I can use SOME of CTA0.