Problem - Ubuntu 16.04.3 & ISPConfig Guest Weekly Freezes under KVM / oVirt

Hi !

I have strange and annoying problem with one VM on oVirt node 4.2 - weekly freezes of Ubuntu 16.04.3 with ISPConfig 3.1 active.
Separate engine PC, not hosted engine.

Ubuntu 16.04.3 LTS (Xenial Xerus), 2 cores allocated, 8 GB RAM (only fraction is being used).
kernel 4.13.0-32-generic
6300ESB Watchdog Timer

Memory ballooning disables, and there are always about 7 GB of free RAM left.
4 VMs active, CPU load on node is low.
Tried several kernel versions, no change.

I can’t trace any problem in the log on Ubuntu guest. Even watchdog timer 6300ESB configured to reset does nothing (what is really strange).
VM stops responding even to pings, VM screen is also frozen.
oVirt engine don’t display IP address anymore, it means ovirt-guest-agent is dead.

VM is in DMZ, and not connected to ovirtmgmt, but rather to bridged Ethernet interface.
in oVirt I have defined network "DMZ Node10-NIC2”.

On CentOS 7 host node (NOT Ubuntu KVM Guest):
cd /etc/sysconfig/network-scripts/
tail ifcfg-enp3s4f1

DEVICE=enp3s4f1
BRIDGE=ond04ad91e59c14
ONBOOT=yes
MTU=1500
DEFROUTE=no
NM_CONTROLLED=no
IPV6INIT=no

Googling doesn’t show anything useful except attempt to change kernel version what I already did.

Any idea how to fix this freeze ?

Thanks in advance for any help.