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Need Help with intermittent user failures to shared folders..

OK, As a network admin with 18 years experience, I am stumped and all help would be appreciated.  Our small company has 3 main sites and small datacenter.  Each site has 1 data server and 1 domain controller in it and then the Datacenter has 2 additional DCs and other shared servers.  Each main site has approximately 30-40 users in them. After many years of working fine, we had a problem last week that moved to another site this week.  The problem is difficult to explain, and I will do my best..

Last Tuesday..... (Site A and the beginning of the issue)
Data server - 2003
Domain Controller - 2003
Clients - Win 7 64
Users coming in Tuesday could logon to their domain machines fine, but could not access shared folders on their local data server.  However, they could access other shared folders located in other sites.  After rebooting the domain controller and Data server access was restored.  

Last Wednesday.... (Datacenter Site)
Domain controllers are 2003 & 2008r2
Users could not authenticate to shared resources in the datacenter including RDS server and sharepoint based sites..  A reboot of the RID master and Infrastructure master restored access.

Today  (Site B)
Domain controller is 2003
Data server is 2008r2
Users came in this morning with the exact same issue as being able to login to their machine and then could not get access to their local server's shred folders, however once again, they could access other site's resources without a problem.  Rebooting both servers resolved issue.

After many years of having this domain in place and operational, we have never had an issue like this before.  I am concerned that I have had this issue 3 times in the past week and have not in the past 10 years.  Any suggestions to possible issues or causes of this to come up all of a sudden..  Also, we are a small network team of 2 and neither of us have touched these servers or made any changes recently..

thanks in advance

Quick notes...  I have run DCDIAG on all DCs and everything passed
I have checked replmon to ensure replication is good
I have verified AD sites and services are all correct
Watch Question

Are the users failing to authenticate using kerberos or NTLM, or something else entirely?  You should be able to see failed authentication attempts from the users or from the servers hosting network resources.  I would expect the servers that users are accessing resources on would show failures as well in the logs.

Can you test and post failed authentication from the domain controller that authenticated the user/server?