safari can´t open the page XXX because the server where this page is located isn´t responding

I get the next error, randomly, when I try to open certain web pages using Safari (& Chrome):

"safari can´t open the page (web address) because the server where this page is located isn´t responding"

In line with previous posts I have tried everything: to reset the network settings, to reset the modem, to modify the DNS to 4.4.4.4 or 8.8.8.8, to configure Ip-v6 to “Link-local only”, to disable firewall & antiviruses, to delete webdata and cookies, and to update OS X Captain. Nothing has solved the problem yet.

I would appreciate to receive some feedback, new ideas for this trouble.

Thank you in advance.