Ok, here's some bizarre observational data.

When I run with PL7 or below the bad temp sensor tends to flip on the plus side more often (eg., +300C) and the other core on the board runs in the 70s - 80s.

When I run with PL8 or PL9 the sensor seems to stay in the negative more often ( -299C currently) and the other core runs in the 60s.

Go figure.  More power = lower temp.

Things go kooky when chips and senors get wonky.

EDIT:  not so sure about this anymore.  It seems to flip back to + without rhyme or reason now.  I may have to just disable the board entirely.  First I'll have to check how much power it's drawing.  I suspect it's now a very inefficient miner electricity wise.

EDIT:  yup, it's been showing +300C or so all day, hence no mining from CTA0 as it's throttling itself.  I wish there was a way to force it NOT to throttle.