Running DSPIC33FJ64GP802 at high temperature
Thanks for everyone's good suggestions.  It turns out that there are two versions of silicon for this chip, and the earlier version is only rated to 140 Celsius.  Sheesh.  This is noted in the "errata sheet."  This is the second time I have been bitten by not reading the errata sheet.  For reasons that are not clear to me, Microchip publishes errata sheets for their parts rather than revising the data sheets.  Anyway the moral is: if you use Microchip, read the errata sheet!

Thanks again.