Domain Controllers Active Directory replication fail in a while until reboot

I have two DCs(Windows 2012 server) in different locations of the same domain, they are connected through permanent VPN connection,both with DNS server installed.

I recently found out that their replications failed, and "repamin /syncall" command only gives RPC error, unless I reboot the PDC,they get back to work again.

I have tested this situation for quite several times when I found the replication failure every morning when I came to my office. Both DNS servers seem to work perfectly with "nslookup" and "ping" command, for I set the primary DNS server to their counterpart and the secondary to itself.

and I am also sure that the RPC service is running normally on both DCs. I've also done some diagnosis by "DCDIAG /TEST:DNS /V /E /F:<filename.log>" which is described in Microsoft Article:

http://social.technet.microsoft.com/wiki/contents/articles/4494.windows-server-troubleshooting-the-rpc-server-is-unavailable.aspx

Though some DNS test failed,but it does not explain why everything goes normally after reboot. and I am not quite sure about the correct DNS configuration for both DCs in my situation, can someone give a simple and clear principle?

Don't know how comes this situation, can someone help?