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Puzzling network outages

I have a very persistent problem with my network that I would like some help in  solving. Several of the workstations periodically lose connectivity to the gateway and Internet. The outages always last less than 4 minutes, and always begin on the :X8 minute of an hour (eg. 12:08, 12:38, 5:48). When the outage is going on, pings into the workstation in question are normal, but ping out to the VPN/Firewall router or to the outside report 'host unreachable' or 'just time out. I believe that pings out to other machines on the net are OK. I have both wired servers and workstations and wireless workstations and other devices. Both wired and wireless devices exhibit the problem.

I have PRTG network monitoring running to diagnose the problem, part of a log is attached from a troubled workstation.

The network uses a Netgear 336G firewall router which handles 2 WANS but just one is in use. There are 2 VOIP devices, a Brother wireless printer, and various windows and mac workstations, and a dlink wireless router configured as an access point.

We have swapped the  336G for a replacement, installed different access points, assigned static instead of DHCP addresses, and temporarily shut down each network device in hope of isolating the problem, but nothing has worked.
4/25/2009 12:20:44 PM 104 ms 14 ms 194 ms 0 % 0 % 100 % 
4/25/2009 12:20:26 PM Error Error Error Error Error 100 % 
4/25/2009 12:19:46 PM Error Error Error Error Error 100 % 
4/25/2009 12:18:50 PM Error Error Error Error Error 100 % 
4/25/2009 12:18:17 PM Error Error Error Error Error 100 % 
4/25/2009 12:17:14 PM 183 ms 15 ms 351 ms 0 % 0 % 100 % 
4/25/2009 12:16:44 PM 129 ms 14 ms 243 ms 0 % 0 % 100 % 
4/25/2009 12:16:14 PM 325 ms 15 ms 635 ms 0 % 0 % 100 %

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1 Solution
Try logging into the router by typing or into the web browser of one of the computers connected to the network. Check the timeout settings, it might be set at 10 minutes (which could be why its happening on the 8's).

Try connecting your modem to a computer and logging in using these IP addresses, the modem might have a timeout set at 10 minutes.

The only other thing I can think of is that there is a program on one of the network computers that is trying to access the network every 10 minutes...but since you said you tried isolating the issue and it didn't work, I would except it to be network settings.
goldbandAuthor Commented:
Thanks Max, that sent me on another investigation that may have been fruitful. My VPN client (home office) was set on automatic connection, and may have been connecting at just that interval. Ive changed the setting and so far the outages do seem to coincide with VPN connection events.

That raises another question.. why would the network be down in this way for a couple of minutes following a VPN connection event? Im using the netgear VPN client in ipsec mode.
It probably takes a few minutes because the network goes offline, and it takes a few minutes for the network computers to 'realize' that there is no network. In other words, if you are not constantly transferring data, then it will take a few minutes for the computer to verify the connection and realize that the VPN has ceased to respond.

After the network computer pings the VPN and tells it wake up, then it will take another few minutes to re-establish a connection.
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