On 04/25/2012 09:14 PM, I wrote:
Other possible causes that have been implicated in clock drift are Xen related issues and NIC problems. I had installed Xen on the server that's giving me problems now, and I had switched to a non-xen kernel in March, but hadn't removed all the xen-related packages and configurations, so it's possible that some of that was causing problems. I've removed all the xen and virtualisation packages and libraries, and I'm going to reboot again in the wee hours to see if that helps.
Well, that seems to have done it! After cleaning up all the remaining xen cruft from the system, and rebooting last night, the clock seems to be rock-solid today. I guess the moral is if you have any of the xen virtualisation libraries or services enabled, you need to be running the xen kernel. I expect the reverse may also be true.