I need to flush dns again and again to make the sites load

If it is the DNS caching on your system, try stopping and restarting the DNS client service in windows.

You can do this by running the following commands:

net stop dnscache
net start dnscache

Then confirm the cache is clear with:

ipconfig /displaydns

Plan B:

Close and reload Chrome. Confirm chrome is closed by checking in the task manager. (many extensions and plugins like to keep Chrome open in the background - like Hangouts, for example.)

Plan C:

Ping the site...

ping www.superuser.com

Pinging superuser.com [198.252.206.140] with 32 bytes of data:
Reply from 198.252.206.140: bytes=32 time=39ms TTL=52
Reply from 198.252.206.140: bytes=32 time=40ms TTL=52
Reply from 198.252.206.140: bytes=32 time=41ms TTL=52
Reply from 198.252.206.140: bytes=32 time=42ms TTL=52

Ping statistics for 198.252.206.140:
    Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
   Minimum = 39ms, Maximum = 42ms, Average = 40ms

Plan D:

An old Windows adage, but when all else fails, reboot.

Useful site: http://www.downforeveryoneorjustme.com/ - check the sites you are trying to access.

Another useful site: http://downdetector.com/ - Reports of outages on services and websites.

Jay M. explain few ways how to resolve this but here is a bit more light on this issue

Google Chrome has it own DNS Cache. You need to restart chrome browser as quickest solution.

Reason is that this cache is build to speedup your HTTP requests. Of course this is the case on MS Windows platform. Misrosoft took some effort to build network profiles to meet some requirements related to LAN you are accessing at the moment. So in case when you are at public place DNS Client windows service will be stopped so it won't cache any DNS resolution.

Why? Because same service is used to cache hosts around you using network discovery service.

From googles stand point it is overhead so they build their own cache in order to lower round trips to DNS resolver which some times may be expensive when you have poor network connectivity.

I struggled for a month, had to flush dns all the time in 3 computers. I changen DNS server to googles DNS server. Primary 8.8.8.8 and secondary 8.8.4.4. This solved my problem. Google how.