Hi I had a similar issue a month or so back where NTP and SNMP packets were, evidently, not reaching their destinations. The issue turned out to be a routing glitch where these packets were being sent to an unused network interface (ie one that was "up" but should have had no traffic on it). As soon as I removed that interface, the "stray" packets started being sent out of the correct interface and NTP was able to resync. When I was researching this, I found an earlier post from someone who'd run into the same issue and he solved it by repeating the "route add default ..." command (from the /etc/rc file). Good Luck! Richard.
To workaround the issue add the following entries to the /etc/rc file:
options timed.enable off
options timed.enable on
This workaround will force the NTP service to initialize again later in the boot