I finally found another report of the same problem:
http://h2np.net/tips/cpudcc.html (Japanese).
This user has the same DFI motherboard, a slightly faster Althlon 64,
and is running the same OS (Debian Sarge). He reports throttling
failure after about 1 hour, the same as my case.
To me this suggests a bug in the chipset/BIOS or powernow-k8 driver
rather than a hardware fault.
--John