The common problem with Windows 2008R2 with DNS server role installed is high RAM usage. This is due to the large number of ports that are opened by DNS: a total of 5000 (2500 UDP IPv4 and 2500 UDP IPv6).
To check socket pool size just run:

Dnscmd /Info /SocketPoolSize

Disabling of IPv6 will solve the problem gently, but still the RAM usage will be high.
If the server acts as a small  DNS server, you can change this number to i.e. 100 by using the command:
Dnscmd /Config /SocketPoolSize 100

And restart DNS service: DNS:

net stop “DNS Server”

net start “DNS Server”