Shared Drives Intermittently Dropping

I have an issue with most of my clients (Win7 and WinXP) accessing the Shared Drives. I have brand new Windows 2012 R2 Server acting as the DC, File Server, DNS, and DCHP. Almost all users are reporting that they cannot access the shared drive while using their computer (not idle). The connection will be re-establish by restart or re-mapping the drives. All the mapped drives are manually connected via command line with persistent enabled (no login script or GPO). Also, whenever they lost connection to the share drives they cannot access the internet.

After surfing the internet, I found some similar issues and applied the changes but still having the issue.
Current Configuration on the server:

• NO external DNS configured on the DC
• All Clients are registered on the DNS
• DNS Interface Set to All IP Addresses
• Dynamic Updates set to Secure
• DNS Properties – Security Tab – Authenticated Users is present with Create all Child Objects
• No Secondary DNS Server
• All Clients does not have any external DNS
• NIC Power Management is set not turn off the device
• Applied this command on all machines "Net Server /autodisconnect -1"

During the initial setup, I set the static IP of the server with proper IP and added an external DNS (4.2.2.2). Another article I came across after doing a lot of research says that if you add an external DNS(during the initial setup), you will have some similar issues with shared drives dropping.

I really believe that the issue is the DNS but I cannot figure out what needs to be change to resolve the issue. On the article I read, he installed a secondary DNS server and that resolves the issue. I want to set a secondary DNS server using one of my server (on the same network and IP) but with little knowledge setting a secondary server I am having issue.

Please HELP!!

Thanks.