Please bear in mind that Intel offers server OS drivers for the DC53427HYE, the D54250WYK supports desktop operating systems only.
Test updating to the latest drivers for your NUC DC53427HYE
File name: HYE-RKE-Server-OS-Driver-Bundle-Oct2014.zip
I already installed those drivers.
Anyway... I think that I found out the real solution.
It's probably a Microsoft Network Drive GPO problem.
Looks like Windows 8.1 applies Network Drive GPO policy every time it refresh it while the user is logged on.
My N.D. GPO were set to "Replace" action, so OS was disconnecting and reconnecting the network drives.
Then any file opened on network drive at that moment, has been disconnected especially Outlook PST files.
I changed my GPO action to "Update" or "Create" and looks like the issue has gone away.
Windows 7 has a different behavior, seem to be it applies Network Drive policy during the login process only.
I'm going to confirm this fix with another post later.