I am having an issue where intermittently the shares and printers on one of my servers will go offline for anywhere from 15 seconds to 5 minutes. During this time when it goes as offline you can still ping the server and open a remote desktop session to it. When this occurs it doesn't happen for everyone. Typically about 30% of my users experience it and it will be a different set of random users each time. When things go offline they see the printers show as offline, if they attempt to access a share it disappears and if they try to open a mapped drive it says it is unavailable.
I have disabled the firewall and replaced the network card in the server going from a Intel to Broadcom card. I've upgraded drivers and this last week I upgraded from Windows 2008 R2 to Windows 2012 R2 but still see the problem. All available Windows Updates have been installed. There are no errors showing in the event logs on either the clients (Windows XP, 7, and 8.1 machines) or the server itself.