"Server not found" problem on FF 39, Win 7

I have Windows 7 premium 64-bit installed and I use a Realtek PCI GBE LAN Network Adapter to connect to the internet. I am not fond of wireless connection so I don't use it at all.
In the last 1 month I've been plagued with problems relating to network connectivity and internet. The networking center shows I'm connected to the internet but when I try to load a page the web browser displays "Server not Found." The only way I can get web pages to load is to refresh them several times. My d/l speed is 10 mbps. I regularily use FF 39 browser, but the same issue occurs on IE as well.

I tried all suggested fixes on Mozilla page, but the problem persists. It is not a firewall or DNS issue, I checked it several times. (Regarding the attachment, I tried "Obtain an IP address automatically" and "Obtain DNS server address automatically", but to no avail.)

After having tried all fixes which I could find online I am inclined to think that this can not be fixed at all. :-/


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OS Version: Microsoft Windows 7 Ultimate, Service Pack 1, 64 bit
Processor: Intel(R) Core(TM)2 Quad CPU Q6600 @ 2.40GHz, Intel64 Family 6 Model 15 Stepping 11
Processor Count: 4
RAM: 4095 Mb
Graphics Card: ATI Radeon HD 3800 Series, 512 Mb
Hard Drives: C: Total - 152617 MB, Free - 43145 MB; D: Total - 238464 MB, Free - 105880 MB; E: Total - 238464 MB, Free - 84493 MB;
Motherboard: ASUSTeK Computer INC., P5K Premium

Run cmd as admin and enter:

Code:

netsh winsock reset

reboot when finished. May help or not, have to see.

You can also try ZenMate as a test, install it for FF (don't register, no need). Make sure it is "on", shield icon top right. And see if you will get "not found" errors.

I am a bit reluctant to resets the Winsock catalog to the default configuration since I've read somewhere that any previously installed LSPs will need to be re-installed, and I do not know how to do it. However, I'll try ZenMate and will post the results.

In the last several hours I did not get "server not found" error, which is a good sign, since it happened every time I opened a new page. However, loading page still takes longer time than it should...

Is this with ZenMate?

Yes.

I said that "Server not found" message does not appear anymore, and it is true. However, as of 10 minutes ago, I started getting this one, instead:

Try turning ZenMate off and then back on. Which country are you connected to? Select one closer to you. (not like there are many to chose from )

ZenMate is just a test in this case, we should try a real solution.
Try changing to OpenDNS and turn off ZenMate. Then run in cmd:
ipconfig /flushdns

I checked Speedtest.net and it seems that ZenMate slows down the connection, sometimes 80%, other times 30-40%.
Can you explain, please, if this command will reset something irreversibly?

So, I should enter 208.67.222.222 and 208.67.220.220, in Primary DNS Server and Secondary DNS Server fields. Here...?