Severe DNS Problem

I am running Macbook Pro 2012 with upgraded RAM and SSD. I bought it with Mountain Lion then , Mavericks and then upgraded into Yosmite . running latest 10.10.5 , for at least one year I am facing every now and then sites opening error , sometimes it does not come for 1 month , sometimes it bugs me everyday and now for one week , I am pretty tired with it. if I open a site in Firefox or in Chrome  , it does not open and got "not found" error in no time, surprisingly if I use Tor browser  , it opens next second, further more torrent clients work like a charm , I clean history , reboot modem and using
sudo killall -HUP mDNSResponder

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in terminal often , its like shooting in dark , sometimes it works , sometimes it does not . I can say I have no idea why most of sites just gone to "not found" and how it can be fixed, I have gone through several articles but none of them seems to work perfectly ok for me.
should I wash away my SSD completely and install a brand new Mavericks copy to solve this ? would it I have to reserver some kind of serial numbers to install a fresh copy ? or I just download Mavericks and run it on a brand new HDD ? I also want to add that since last some updates I have found that my RAM got exhausted a lot , I was using 8 GB (2x 4GB) but last month one RAM is fried and on replacement , I have found a single 4GB RAM exhausted most of the time , free memory status never went up more than 800MB while not a single app is opened except Finder.
Amit KumarAsked:
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James NewportCommented:
Have you tried using different DNS servers to your ISP's? Try using googles and

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It sounds like you might have some DNS malware on your computer. Download and run a free AntiVirus program like Avast: and see what it finds.

Also, check your network prefs to see what DNS servers you are using. You could try adding the Google DNS servers manually:
Wow, Relyonit. You beat me by 4 seconds!
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Also, to see what is using up RAM, open up Activity Monitor (found in /Applications/Utilties ) and sort by the CPU% column.
Amit KumarAuthor Commented:
@strung ,  I am using Avast Free , there is one another noticeable thing I have under network pane is , I use US server, GB server and JP Server kind of vpn settings leds to,, and though there are inactive but non delectable. I just remember I have had  Pure VPN  installed but never used.

Memory : biggest memory eaters are Firefox with 6-7 tabs open 700mb, kernal_task 500mb around, Finder 135mb, Avast 100+mb and rest of them tini-tiny from 60mb to 1mb .
I would suggest that you delete all your VPN's and see if the problem goes away. If it does, add them back in one by one as necessary.
Amit KumarAuthor Commented:
there is no VPN installed however, if you are talking about US server, GB server and JP Server kind of vpn settings lied in my network pane, there is no option active to delete , that "-" sign not highlighted
Google is not the only open DNS provider.  I tend to vary them to not get stuck with google's tracking every single detail.
Comodo Secure DNS
OpenDNS Home
DNS Advantage
Norton ConnectSafe
Hurricane Electric	 

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