That was the second thing I tried after resetting the USB bus.  But thanks anyway.  ;-)

The only thing that seems to work (mostly) reliably is to blow compressed air towards the contact between the hashing chips and the cooler.  So, somethiing is horked with the temperature sensor that lets it get into the boot sequence - either it's got too much leftover thermal paste on it or it has an offset of some sort.  I did make it almost spotless when I cleaned up the paste and converted to thermal pad, but...