[CentOS] ntpd heavy CPU consumption

CentOS 6.4, x86_64.

ntpd on one of my systems has started consuming 66% of one core, although 
it appears to be functioning correctly otherwise. No pertinent logs. Of 
course, nothing was changed :) I've seen this before many times, but 
usually the CPU consumption falls back to normal within a day or so, but 
this has been going on for several weeks now. Stopping and restarting ntpd 
makes no difference. Reinstalling ntpd makes no difference. The ntpd.conf 
file is the same as on 200+ other systems, all of which work normally. 
Anyone seen this before?

Steve