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Clients Intermittent Connectivity to Internet

So I've got 3 clients out of 8 that randomly are losing their connectivity outside of our network.  A little architecture:
All PC's run back to a Linksys 24-port managed switch
DHCP handled through Cisco ASA
DNS is configured with the SBS 2011 as the primary and as the secondary right now
Problem is occurring with static or DHCP addresses
Clients will connect for a time, and then suddenly only show local connectivity.
The switch ports are good, connections are intact, PC's in the same office are running just fine on static or DHCP with same configurations, all outbound internet traffic at F/W is permitted.

Anyone?  I'm at a loss.
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Sometimes resetting the adapter helps, but really the only commonality that I can see at all is that the SBS is new.  We haven't had this issue previously.
Also, consider letting your SBS server handle DHCP since it apparently is handing DNS. I use this method and DHCP on all computers. Whenever a person moves a laptop out of the office, there is no difficulty with outside connectivity.

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Unfortunately, we're running two different networks and the F/W needs to handle the DHCP.  Microsoft assisted with the initial setup so this hasn't been an issue until recently with no changes.  If I static the IP's of the machines outside of the SBS and FW (to a separate 10/Net for our phone system), then the machine gets out fine.  It's only when connecting through the domain.

It can ping itself, it can ping by IP and name the SBS and the F/W, gateway.
Since you have a managed switch:Use two vlans and let SBS do DHCP and DNS!
If SBS detects a DHCP online it shuts down it's ownDHCP and you getsome strange behaviour (Also none of the wizards will work the way they should)
Hope that helps,
Check the logs, show ARP commands on your managed switch, and/or use WireShark to check for duplicate addresses.

Also, depending if your two networks are able to interVLAN route between eachother.  If so, then your SBS can be configured as the DHCP server and your L3 interVLAN routing device (either L3 switch, ASA Firewall, or other router) can be configured with DHCP relay settings i.e. IP helper-address on the LAN interfaces to point to the SBS to relay DHCP and other domain traffic.

You indicated 'switch ports are good' so just to confirm here are some other considerations. The switch network interfaces experiencing high rate of errors due to bad cables, bad switch ports/NICs, speed and/or duplex mismatch, interface resets, etc...  You should be able to look at your switch port statistics and/or other switch management options.  Note most switches these days have a built-in error disable feature that automatically disables ports when it has reached a certain threshold of errors.   Note speed and duplex mismatches are a common problem and can be identified by FCS, CRC, Giants, Runts, and other common errors as well as collisions.

FYI... Although your network topology sounds kind of simple. Spanning-tree, if enabled, could put an interface in a blocking state; as a result it eutt
As a result of a switching loop.
Thanks for all of the inputs.  This seems like it could be the most reasonable problem.  Our ASA appears to be licensed for 10 users.  With all of the smartphones and tablets now being configured and accessing our wireless access point, they're getting internal addresses which would count against that license number.  What might also support this is that I can set the PC's to an IP outside of the domain (the IP Phone network) and they access the internet just fine (albeit without domain resources).  <br /><br />I'll followup with Cisco on this.  Seems like a promising place to start.