2012 R2 DC not reachable

Hi,

my business and I service about 300 customers in the small business range, mostly with pure Microsoft environments.

Our so far only two customers who run no servers other than Windows Server 2012 R2 experience this problem:

Every now and then, every other day or less frequent, logon into RDS is not possible because no domain controller is available.  DNS service does not work even though the service is running.  The AD member servers log frequent events relating to no DC available.  The DC logs no events, feels just fine, cannot resolve any DNS queries and sits there in ignorant bliss.
Restart of the service does not help, I have to reboot the entire machine .

Both customers run VMware, one has Exchange, the other does not.   Both customers have only one single DC. One domain, one tree.  
Eventlog does not record anything of any help.  

Many other customers run Server 2012 R2 for some services and 2008 R2 for others.  They don't report anything like this.

Does anybody have a clue what might be happening there?  Has anyone experienced something like that?

Thanks,
Ralph