This is also happening to me  EXACTLY as you...

How can I reproduce this bug? This way:

I have a  Windows 2008 Raw Image UP in my openstack cloud (ESSEX).
(with a flatdhcp multi-host configuration)

When I create a New instance with this W2008 image everything comes up smoothly , Everything works as it should.. but 5 - 10 minutes later.. .something happens...:

Windows loses Connectivity via rdesktop.  I can see the instance running on the compute machine. And I can connect to it via VNC.

After analyzing  Windows LOGs I discovered that after Windows Synchronizes the clock with the network clock Windows disables and enables the local network connection (nic).
But it is done very very fast.

 Here, this is when my windows instance loses the IP!! The DHCP Ip address is set to:  169.254.141.59
Why?!?  I don't know

I don't know if restarting the network interface is a normal process on the windows world but.. Doing this manually on windows: it has(after 20-100 tries) the same effect, DHCP address gets lost.

and looking at the /var/log/syslog  file I have exactly the same error as you:

Feb 15 12:54:49 osqcomp06 2013-02-15 12:54:49 DEBUG nova.dhcpbridge [-] Called 'del' for mac 'fa:16:3e:5d:88:fe' with ip '10.5.96.21' from (pid=26734) main /usr/bin/nova-dhcpbridge:113
Feb 15 12:54:49 osqcomp06 2013-02-15 12:54:49 DEBUG nova.rpc.amqp [req-060372df-c625-491e-8a59-7b037b2f7c2a None None] Making asynchronous cast on network.osqcomp06.kloudopen.com... from (pid=26734) cast /usr/lib/python2.7/dist-packages/nova/rpc/amqp.py:351
Feb 15 12:54:49 osqcomp06 2013-02-15 12:54:49 DEBUG nova.rpc.amqp [req-060372df-c625-491e-8a59-7b037b2f7c2a None None] Pool creating new connection from (pid=26734) create /usr/lib/python2.7/dist-packages/nova/rpc/amqp.py:60
Feb 15 12:54:49 osqcomp06 2013-02-15 12:54:49 INFO nova.rpc.common [req-060372df-c625-491e-8a59-7b037b2f7c2a None None] Connected to AMQP server on osqctrl01:5672
Feb 15 12:54:49 osqcomp06 2013-02-15 12:54:49 DEBUG nova.rpc.amqp [-] received {u'_context_roles': [u'admin'], u'_context_request_id': u'req-060372df-c625-491e-8a59-7b037b2f7c2a', u'_context_read_deleted': u'no', u'args': {u'address': u'10.5.96.21'}, u'_context_auth_token': '<SANITIZED>', u'_context_is_admin': True, u'_context_project_id': None, u'_context_timestamp': u'2013-02-15T18:54:49.242450', u'_context_user_id': None, u'method': u'release_fixed_ip', u'_context_remote_address': None} from (pid=452) _safe_log /usr/lib/python2.7/dist-packages/nova/rpc/common.py:160
Feb 15 12:54:49 osqcomp06 2013-02-15 12:54:49 DEBUG nova.rpc.amqp [req-060372df-c625-491e-8a59-7b037b2f7c2a None None] unpacked context: {'user_id': None, 'roles': [u'admin'], 'timestamp': '2013-02-15T18:54:49.242450', 'auth_token': '<SANITIZED>', 'remote_address': None, 'is_admin': True, 'request_id': u'req-060372df-c625-491e-8a59-7b037b2f7c2a', 'project_id': None, 'read_deleted': u'no'} from (pid=452) _safe_log /usr/lib/python2.7/dist-packages/nova/rpc/common.py:160
Feb 15 12:54:49 osqcomp06 2013-02-15 12:54:49 DEBUG nova.network.manager [req-060372df-c625-491e-8a59-7b037b2f7c2a None None] Released IP |10.5.96.21| from (pid=452) release_fixed_ip /usr/lib/python2.7/dist-packages/nova/network/manager.py:1254

The IP is RELEASED...
Why?
I'm still looking for answers ..

Ahh and after that I have the next LOG repeated many times(at least once per minute):

Feb 15 13:32:24 osqcomp06 dnsmasq-dhcp[12632]: DHCPREQUEST(br100) 10.5.96.21 fa:16:3e:5d:88:fe 
Feb 15 13:32:24 osqcomp06 dnsmasq-dhcp[12632]: DHCPACK(br100) 10.5.96.21 fa:16:3e:5d:88:fe testx-030
Feb 15 13:32:24 osqcomp06 2013-02-15 13:32:24 DEBUG nova.dhcpbridge [-] Called 'old' for mac 'fa:16:3e:5d:88:fe' with ip '10.5.96.21' from (pid=31227) main /usr/bin/nova-dhcpbridge:113


Thanks in advance
gatuus