A puzzle with internet time and NIST time

[Paul E Condon <pecondon@mesanetworks.net>]

I've been running Debian for many years. During most of those years I
have had a SkyScan(tm) 'Atomic Clock' on the wall near my Debian
desktop computer. The physical computer has changed over the years,
but not my using Debian, or my Atomic Clock. Until a few weeks ago,
they always displayed the same time, once I had correctly installed
the chrony or the ntp package. But some time recently they started
disagreeing by about 18 seconds with Debian/Squeeze running ahead, yes
ahead, of the Atomic Clock which is supposedly getting its signal by
radio direclty from the NIST transmitter. How can this be???? I've
reinstalled the software, but that didn't change the behavior, except
for a brief period when the local computer clock resynchoronized with
the time server pool. There's nothing remarkable about my hardware.
An aging dual core pentium, marketed by HP and running Squeeze with
Gnome desktop. Ideas?

-- 
Paul E Condon           
pecondon@mesanetworks.net