Computers experience dropouts on Windows server 2012 domain

I am pressed for time on resolving this issue, so it is definitely worth 500 points to me to be able to get it resolved. I just completed a migration from Windows SBS 2003, TO  Windows Server 2012 with Essentials installed. Out of approximately 40 Windows 7 Pro client workstations on the domain that are functioning perfectly well, I have five workstations that are constantly dropping off the network since the migration has been completed. The five workstations that are experiencing the dropouts, are coming off of a 3Com SuperStack 24 port managed switch, which is coming off of another of the same type managed switch coming off the new server. I had changed out the suspected switch, with an unmanaged switch this afternoon and still have the same issue. Within 15 to 30 minutes, dropouts began. Again, all other workstations on the domain or functioning correctly with no problems. Again this issue did not exist at all until installing and migrating onto the new server.
Houston BlancettAsked:
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Cliff GaliherCommented:
Just so I can get a better sense of the issue, "dropping off" is rather vague. What exotically are you seeing as symptoms? What verification steps have you taken? What troubleshooting steps have you taken? Why did you suspect the switch? And are there other machines on this switch?
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Houston BlancettAuthor Commented:
Yes I should clarify dropping off. They can be online with access to all mapped drives without any problems then for some reason they will lose connectivity drives that are mapped will become unavailable, and they will lose Internet access.
We suspected the switch because all of the PCs that were having these problems were connected to that particular switch. The only machines on the switch for the five PCs that are having these issues.
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Cliff GaliherCommented:
Are both switches in the same area/server room/data closet? Do all machines drop off at the same time or is each of the five failing independently of the others? Does powering off the client and rebooting fox the issue? Or the switch? Or the server? Or do they come back on their own?

Three things I'd do as troubleshooting steps. First test ping the server while client machines are working. Then when they drop off, test ping again. Second, compare IPconfigs from before the drop to during the drop, and to a machine that doesn't fail. Maybe a rogue DHCP server or bad DNS settings. And third, plug a machine that fails into the other switch (if possible) and a machine that always behaves into the suspect switch.

Right now troubleshooting is about eliminating potential causes as much as it is proving a suspected cause.
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tolinromeCommented:
If its only 5 clients and those clients be moved to another switch until the problem is resolved?
Has anything changed within the DHCP settings scope ?
It just doesn't make sense that up grading a server would effect something on the network switch or the network layer.
Has anything else been changed or touched during the upgrade?
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Houston BlancettAuthor Commented:
One thing I've just noticed...if I open up network connections on one of the PC's in question and right-click and look at "status" it's showing  the correct DHCP server IP, and for DNS 1...but for DNS 2, it seems to be showing the IP address of the old (non-demoted) windows abs 2003 server.
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Cliff GaliherCommented:
Which would also show up n ipconfig, which is why I listed it as a troubleshooting step. You can fix that particular issue by editing your DHCP scope options.
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Adam RayCommented:
If the issue persists after you've verified DNS is fully operational (or set it so that workstations only use the 2012 server for DNS)...

Try disabling IPv6 on the affected workstations. It's not actual "fix," but it could help narrow down the cause. (IPv6 and/or DNS resolution for IPv6 can cause problems in some situations.)
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Daniel BlackmoreInfrastructure EngineerCommented:
When you demoted the old 2003 SBS server, did you leave it online? You may not have disabled the old DHCP scopes.

Go over all your scopes, check the statistics for any errors or bad addresses, verify that all the DHCP options are correct
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Houston BlancettAuthor Commented:
Sorry for delay...got caught up on another job. DHCP scope is correct. I did disable the old DHCP scope when I demoted the SBS 2003 server.

I checked in DNS and there are some entries that are still listing the old SBS server as a DNS server complete with the (now inactive) IP address of the old server.

DNS, New Server, Forward Lookup Zone, old server (showing as an Alias (CNAME)

DNS, New Server, Forward Lookup Zone, Domain Name, old server (showing as an Alias (CNAME)

DNS, New Server, Forward Lookup Zone, Domain Name, _msdcs, old server showing as a name server

At this point moved 5 pc's top another switch and for the most part, they are stable however customer tells me new work super fast and all services working first thing in the morning until every pc starts logging in - then network slows and pc's start losing connectivity.
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Houston BlancettAuthor Commented:
Following the troubleshooting suggestion I determined that the issue was actually a RS6000 that users were log gin into that was still pointed to the IP of the demoted and retired server.
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