Barring the possibility that your powerline frequency is erratic, I'm
inclined to accept Mr Cook's explanation -- that these clocks have poor
powerline filtering, and spikes get through to trip the counter. You might
try putting a ferrite choke on the line.

The LED clock has become uncommon, if only because it doesn't lend itself to
cordless operation. I keep one in the bedroom for those occasions when I
need a loud alarm, but it's not atomically controlled.

This one looks interesting. It //claims// to always display the correct time
and adjust for DST -- which would require access to a stable time source.

http://www.amazon.com/Chaney-Instrum...5811&sr= 1-22

You might look for atomic clocks using vacuum-fluorescent displays.