LinuxFloat

My CentOS 5.7 server is frequently down in recent days.I checked every parts without error except for the /var/log/messages.There are a bunch of errors in the messages log file as below.How can I fix this problem?Please help.

Feb 19 04:02:58 host syslogd 1.4.1: restart.
Feb 19 04:14:56 host ntpd[1801]: time reset +0.341016 s
Feb 19 04:14:56 host ntpd[1801]: frequency error 512 PPM exceeds tolerance 500 PPM
Feb 19 04:18:45 host ntpd[1801]: synchronized to LOCAL(0), stratum 10
Feb 19 04:18:45 host ntpd[1801]: frequency error 512 PPM exceeds tolerance 500 PPM
Feb 19 04:19:48 host ntpd[1801]: synchronized to 208.83.212.8, stratum 2
Feb 19 04:22:01 host kernel: md: syncing RAID array md0
Feb 19 04:22:01 host kernel: md: minimum _guaranteed_ reconstruction speed: 1000 KB/sec/disc.
Feb 19 04:22:01 host kernel: md: using maximum available idle IO bandwidth (but not more than 200000 KB/sec) for reconstruction.
Feb 19 04:22:01 host kernel: md: using 128k window, over a total of 104320 blocks.
Feb 19 04:22:01 host kernel: md: delaying resync of md1 until md0 has finished resync (they share one or more physical units)
Feb 19 04:22:02 host kernel: md: md0: sync done.
Feb 19 04:22:02 host kernel: md: syncing RAID array md1
Feb 19 04:22:02 host kernel: md: minimum _guaranteed_ reconstruction speed: 1000 KB/sec/disc.
Feb 19 04:22:02 host kernel: md: using maximum available idle IO bandwidth (but not more than 200000 KB/sec) for reconstruction.
Feb 19 04:22:02 host kernel: md: using 128k window, over a total of 2096384 blocks.
Feb 19 04:22:02 host kernel: RAID1 conf printout:
Feb 19 04:22:02 host kernel:  --- wd:2 rd:2
Feb 19 04:22:02 host kernel:  disk 0, wo:0, o:1, dev:xvda1
Feb 19 04:22:02 host kernel:  disk 1, wo:0, o:1, dev:xvdb1
Feb 19 04:22:33 host kernel: md: md1: sync done.
Feb 19 04:22:33 host kernel: RAID1 conf printout:
Feb 19 04:22:33 host kernel:  --- wd:2 rd:2
Feb 19 04:22:33 host kernel:  disk 0, wo:0, o:1, dev:xvda2
Feb 19 04:22:33 host kernel:  disk 1, wo:0, o:1, dev:xvdb2
Feb 19 04:22:57 host ntpd[1801]: synchronized to 129.128.5.210, stratum 1
Feb 19 04:35:16 host ntpd[1801]: time reset +0.378633 s
Feb 19 04:35:16 host ntpd[1801]: frequency error 512 PPM exceeds tolerance 500 PPM
Feb 19 04:38:37 host ntpd[1801]: synchronized to LOCAL(0), stratum 10
Feb 19 04:38:37 host ntpd[1801]: frequency error 512 PPM exceeds tolerance 500 PPM
Feb 19 04:39:43 host ntpd[1801]: frequency error 512 PPM exceeds tolerance 500 PPM
Feb 19 04:40:48 host ntpd[1801]: synchronized to 129.128.5.210, stratum 1
Feb 19 04:53:22 host ntpd[1801]: synchronized to 216.194.70.2, stratum 2
Feb 19 04:55:52 host ntpd[1801]: synchronized to 129.128.5.210, stratum 1
Feb 19 05:01:53 host ntpd[1801]: time reset +0.272259 s
Feb 19 05:01:53 host ntpd[1801]: frequency error 512 PPM exceeds tolerance 500 PPM
Feb 19 05:05:58 host ntpd[1801]: synchronized to LOCAL(0), stratum 10
Feb 19 05:05:58 host ntpd[1801]: frequency error 512 PPM exceeds tolerance 500 PPM
Feb 19 05:07:01 host ntpd[1801]: synchronized to 129.128.5.210, stratum 1
Feb 19 05:24:59 host ntpd[1801]: time reset +0.193815 s
Feb 19 05:24:59 host ntpd[1801]: frequency error 512 PPM exceeds tolerance 500 PPM
Feb 19 05:28:14 host ntpd[1801]: synchronized to LOCAL(0), stratum 10
Feb 19 05:28:14 host ntpd[1801]: frequency error 512 PPM exceeds tolerance 500 PPM
Feb 19 05:29:20 host ntpd[1801]: synchronized to 129.128.5.210, stratum 1
Feb 19 05:38:45 host ntpd[1801]: frequency error 512 PPM exceeds tolerance 500 PPM
Feb 19 05:50:50 host ntpd[1801]: time reset +0.277485 s
Feb 19 05:50:50 host ntpd[1801]: frequency error 512 PPM exceeds tolerance 500 PPM
Feb 19 05:54:25 host ntpd[1801]: synchronized to LOCAL(0), stratum 10
Feb 19 05:54:25 host ntpd[1801]: frequency error 512 PPM exceeds tolerance 500 PPM
Feb 19 05:55:30 host ntpd[1801]: frequency error 512 PPM exceeds tolerance 500 PPM
Feb 19 05:56:35 host ntpd[1801]: synchronized to 129.128.5.210, stratum 1
Feb 19 06:13:02 host ntpd[1801]: time reset +0.339241 s