ntp error: time sync and frequency error

Hi Expert,

One on my server keep logging this error below, how to fix this issue?
Thank you.
Reggy.

pluton02:/home/reggy[ 61 ] --> sudo grep "ntpd" /var/log/messages | tail
Oct 6 00:26:18 pluton02 ntpd[6092]: time reset -6.588403 s
Oct 6 00:26:18 pluton02 ntpd[6092]: frequency error -512 PPM exceeds tolerance 500 PPM
Oct 6 00:27:37 pluton02 ntpd[6092]: synchronized to 163.181.73.251, stratum 4
Oct 6 00:41:37 pluton02 ntpd[6092]: time reset -3.839520 s
Oct 6 00:41:37 pluton02 ntpd[6092]: frequency error -512 PPM exceeds tolerance 500 PPM
Oct 6 00:42:57 pluton02 ntpd[6092]: synchronized to 163.181.224.251, stratum 4
Oct 6 00:48:23 pluton02 ntpd[6092]: synchronized to 163.181.34.251, stratum 4
Oct 6 00:56:51 pluton02 ntpd[6092]: time reset -6.845404 s
Oct 6 00:56:51 pluton02 ntpd[6092]: frequency error -512 PPM exceeds tolerance 500 PPM
Oct 6 00:58:10 pluton02 ntpd[6092]: synchronized to 163.181.224.251, stratum 4

pluton02:/home/reggy[ 62 ] --> sudo grep "ntpd" /var/log/messages | head
Oct 3 04:15:27 pluton02 ntpd[6092]: time reset -8.076310 s
Oct 3 04:15:27 pluton02 ntpd[6092]: frequency error -512 PPM exceeds tolerance 500 PPM
Oct 3 04:16:46 pluton02 ntpd[6092]: synchronized to 163.181.224.251, stratum 4
Oct 3 04:29:46 pluton02 ntpd[6092]: synchronized to 163.181.73.251, stratum 4
Oct 3 04:30:49 pluton02 ntpd[6092]: time reset -3.182774 s
Oct 3 04:30:49 pluton02 ntpd[6092]: frequency error -512 PPM exceeds tolerance 500 PPM
Oct 3 04:32:00 pluton02 ntpdate[13318]: the NTP socket is in use, exiting
Oct 3 04:32:00 pluton02 ntpd: failed
Oct 3 04:32:08 pluton02 ntpd[6092]: synchronized to 163.181.34.251, stratum 4
Oct 3 04:46:04 pluton02 ntpd[6092]: time reset -6.129951 s