That sounds like a faulty sensor - those little bugs are almost never reliable, in any platform.
Thanks for the quick reply. I've put together quite a few builds over the years, but never had a CPU temp sensor that simply read 0C or 1C, then jumped up scale to the teens, twenties, and back down again. I found a review here: http://www.overclockers.com/amd-trinity-a10-5800k-apu-review
that seems to be describing what I'm seeing.