I have tried using Google DNS and the one provided with DHCP. At some point my PC (Windows 7) stops resolving domain names, but DNS server is perfectly pingable. What can be the reason? Thanks!

Edit: It is really weird. It can stop and start working in few seconds. The problem is that DNA requests are timing out, and the problem is that the DNS server is pingable at the same time. I can't understand how this could be possible and what might be an issue.

C:\Windows\System32\drivers\etc>nslookup google.com
DNS request timed out.
    timeout was 2 seconds.
Server:  UnKnown
Address:  8.8.8.8

Non-authoritative answer:
Name:    google.com
Addresses:  173.194.78.102
          173.194.78.101
          173.194.78.139
          173.194.78.113
          173.194.78.100
          173.194.78.138


C:\Windows\System32\drivers\etc>nslookup google.com
DNS request timed out.
    timeout was 2 seconds.
Server:  UnKnown
Address:  8.8.8.8

DNS request timed out.
    timeout was 2 seconds.
DNS request timed out.
    timeout was 2 seconds.
*** Request to UnKnown timed-out