Cisco 871 - clients periodically do not resolve dns. How to fix?

Question

Domain Name System

Good afternoon.
Faced strange dns work on network computers (small network, up to 15 computers).
Periodically stop resolving dns, while some of the resources are working.
It happened several times in a month, but if problems begin, then this is for the whole day. That is, everything can work for a week, or maybe, like today, it can be repeated every 15 minutes. It is treated by rebooting 871, but not for long. I did not touch the cisco 871 configuration for a month.
There was a problem, the name does not resolve:

C:\Users\User>ping mail.google.com
При проверке связи не удалось обнаружить узел mail.google.com.
Проверьте имя узла и повторите попытку.

Ok, we go via ssh to 871, we try from there:

router#ping mail.google.com

Translating "mail.google.com"...domain server (8.8.8.8) [OK]

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 173.194.113.213, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 24/24/28 ms

Now again from the computer:

C:\Users\User>ping mail.google.com

Обмен пакетами с googlemail.l.google.com [173.194.113.213] с 32 байтами данных:
Ответ от 173.194.113.213: число байт=32 время=19мс TTL=59
Ответ от 173.194.113.213: число байт=32 время=24мс TTL=59
Ответ от 173.194.113.213: число байт=32 время=26мс TTL=59
Ответ от 173.194.113.213: число байт=32 время=27мс TTL=59

Статистика Ping для 173.194.113.213:
    Пакетов: отправлено = 4, получено = 4, потеряно = 0
    (0% потерь)
Приблизительное время приема-передачи в мс:
    Минимальное = 19мсек, Максимальное = 27 мсек, Среднее = 24 мсек

What did I set up wrong?

cisco configuration

version 12.4
no service pad
service timestamps debug datetime msec
service timestamps log datetime msec
no service password-encryption
service internal
!
hostname router
!
boot-start-marker
boot-end-marker
!
!
no aaa new-model
clock timezone Moscow 3
clock summer-time Moscow recurring last Sun Mar 2:00 last Sun Oct 2:00
no ip gratuitous-arps
ip cef
!
!
no ip dhcp use vrf connected
ip dhcp excluded-address 10.1.2.1 10.1.2.99
ip dhcp excluded-address 10.1.2.200
!
ip dhcp pool LAN
network 10.1.2.0 255.255.255.0
dns-server 10.1.2.1
default-router 10.1.2.1
!
!
ip domain name some.name
ip name-server 8.8.8.8
ip name-server 4.2.2.2
ip multicast-routing
ip ssh version 2
!
multilink bundle-name authenticated
!
archive
log config
logging enable
hidekeys
!
!
interface FastEthernet0
!
interface FastEthernet1
!
interface FastEthernet2
!
interface FastEthernet3
!
interface FastEthernet4
ip address dhcp client-id FastEthernet4
ip nat outside
ip virtual-reassembly
duplex auto
speed auto
!
!
interface Vlan1
ip address 10.1.2.1 255.255.255.0
ip nat inside
ip nat enable
ip virtual-reassembly
!
ip local pool VPN 10.1.2.50 10.1.2.75
ip default-gateway 10.1.2.1
ip route 0.0.0.0 0.0.0.0 FastEthernet4 dhcp
!
!
no ip http server
no ip http secure-server
ip dns server
!
!
!
!
control-plane
!
!
line con 0
logging synchronous
no modem enable
line aux 0
line vty 0 4
login local
transport input ssh
!
scheduler max-task-time 5000
ntp clock-period 17175064
ntp master
ntp server 67.215.65.132
ntp server 91.236.251.12
end

Answer 1

Change dns from Google to Yandex 77.88.8.8 and the second from root servers that are closer to you