DNS gives intermittent error

Hi,
I have setup my DNS which has our abc domain. It also hosting other domain like abc.com.

We have proxy server as our gateway and I add our primary and secondary dns (in proxy server) to point to our dns under TCPIP -> DNS Service Search Order and it has a bit of problem. I can browse yahoo or hotmail but if I browse certain web site like techrepublic it gives error (the proxy gives error) because our staff install winsock proxy client to access internet. But if I change the dns service search order to point to our ISP DNS, it works fine.

How come my DNS allow only certain web site like yahoo, hotmail, but disallow techrepublic?

Can anyone help me here.
Thanks
uji