Linux webserver reboot issue

Question

I've webserver, it works fine when there is no load into it. when I've load onto it and try to restart httpd server gets reboot itself or sometimes it automatically get reboot

It is Xen HVM with OS CentOS 6.2. I'm unable to trace the fault, even I've not found anything in /var/log/message or dmesg. Any help in this regard

restarting httpd only not whole VM

here are logs for this particular VM

domid: 4
config qemu network with xen bridge for  tap4.0 br0
Using file /mnt/Servers/webserver/webserver-cent62.img in read-write mode
Watching /local/domain/0/device-model/4/logdirty/cmd
Watching /local/domain/0/device-model/4/command
Watching /local/domain/4/cpu
char device redirected to /dev/pts/1
qemu_map_cache_init nr_buckets = 10000 size 4194304
shared page at pfn feffd
buffered io page at pfn feffb
Guest uuid = 6d0055e3-0d25-8c26-f7a2-45b675c28ec3
Time offset set 0
char device redirected to /dev/pts/2
xen be: console-0: xen be: console-0: initialise() failed
initialise() failed
populating video RAM at ff000000
mapping video RAM from ff000000
Register xen platform.
Done register platform.
platform_fixed_ioport: changed ro/rw state of ROM memory area. now is rw state.
xs_read(/local/domain/0/device-model/4/xen_extended_power_mgmt): read error
Log-dirty: no command yet.
vcpu-set: watch node error.
xen be: console-0: xen be: console-0: initialise() failed
initialise() failed
xs_read(/local/domain/4/log-throttling): read error
qemu: ignoring not-understood drive `/local/domain/4/log-throttling'
medium change watch on `/local/domain/4/log-throttling' - unknown device, ignored
xen be: console-0: xen be: console-0: initialise() failed
initialise() failed
xen be: console-0: xen be: console-0: initialise() failed
initialise() failed
cirrus vga map change while on lfb mode
mapping vram to f0000000 - f0400000
platform_fixed_ioport: changed ro/rw state of ROM memory area. now is rw state.
platform_fixed_ioport: changed ro/rw state of ROM memory area. now is ro state.
Unknown PV product 3 loaded in guest
PV driver build 1
region type 1 at [c100,c200).
region type 0 at [f3001000,f3001100).
squash iomem [f3001000, f3001100).