Server and clients lockup daily between 3:50PM and 4:10PM

I have a network problem that I just can't seem to resolve.  I am consulting for a client that has a network that consists of two servers and aprox 20 workstations.  Server 1 is running Windows 2003 server standard, and is the Domain Controller.  Server 2 is Windows 2003 64 bit and is running Exchange 2007, plus SQL standard.  Clients are all running Windows XP pro.

Since the network was created, client has found that between 3:50PM - 4:10pm, the server and PC's will "pause" or lockup, and then after a couple of minutes resume working.  No data seems to be lost, and machines generally don't crash, and the error happens daily around 4PM.

We have looked into scheduled tasks, checked event logs - there are no entries recorded in the event log during the lockup period, and checked for virus' and spyware.  I have connected a PC to the network that isn't part of the domain, and it doesn't have any problems.

I have confirmed that group policy refreshes every 15-30 minutes, plus time sync happens every 90 minutes too.  Windows updates don't seem to happen during this period either.  We have connected a traffic monitoring program called SolarWinds, and less than 1% or the gigabit network is utilized during the freeze up period.

Any ideas anyone?
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Hypercat (Deb)Commented:
I would check further on your servers for utilities or maintenance tasks that might be running at that time, particularly anything having to do with Active Directory replication or Exchange 2007 maintenance tasks.  Do you have only one domain controller, or is the second server also running AD? Have you checked for automatic antivirus scans, defragging jobs or something similar that might not show as a scheduled task but is nevertheless scheduled to run at a certain time?
plythallAuthor Commented:
So far we can't find anything obvious like virus scanning, or a defrag.  The network has only 1 domain controller, and the Exchange server has a link to Active Directory, but not installed on the server.  I thought of replication, but everything I see so far points to AD replication ever 15 minutes or so, not once per day.

Other oddity is if it was something like defrag, I would expect to see something in the event log, but the log stops recording during the lockup period.
Hypercat (Deb)Commented:
Are they running some third party app - maybe a SQL database - that could be trying to do some kind of updating once a day?  

Ever listen to "Car Talk" on NPR?  This is a "Stump the Chump" question if I ever heard one ;-)

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Hypercat (Deb)Commented:
Try taking a look at this article I ran across today:
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Windows Server 2003

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