It sounds like crappy DNS server(s). (They are the internet “phonebook” that looks up the IP addresses when you ask for a website by URL.)
You can use any DNS servers you choose. And normally, you can specify a list of them to use. If the first is down, or doesn’t respond, the next is tried, and so forth.
We had a similar issue here. I had installed Comodo Internet Security, which replaced the IP addresses for the primary and secondary DNS servers (which were set to earthlink.net DNS servers as they are our ISP,) … and replaced them with two Comodo DNS secure servers that do filtering of bad websites (phishing, fraud, and sometimes explicit content, etc.)
The problem is, that the Comodo secure DNS servers are notorious for going down, and not being fixed for hours. If they are the only ones in your list, you cannot reach any site.
The solution was to insert a bunch of other free DNS servers into the list (with the Comodo DNS IPs at the very end.)
There are at 4 levels of servers (that I’ve found.) Those that do no blocking, those that block insecure sites, those that block insecure+“mature” sites, those that block the all the previous + “porno”. Different servers call them by different names. (Ie, “FamilyShield”, etc.)
So, it seems that your ISP’s DNS servers may be unreliable.
OpenDNS has public non-blocking and “FamilyShield” DNS (a primary and alternate for each.)
Norton also has public secure DNS servers at 3 levels of blocking.
Google has public DNS with no blocking.
There are likely other companies that offer public DNS. Search on “public DNS”.
Other security product companies likely offer secure DNS. Just check to be sure they are public.
The OpenDNS instruction page tells how to add/edit DNS servers under OSX. (And has “purty” pictures.)
On PC, to add or change DNS servers:
Control Panel > Network and Sharing Center > Change adapter settings
Choose either the wired or wireless adapter (whichever one you use.)
Right-click it, choose “Properties” (or double-click it and click “Properties” button on the Status dialog.)
In the list of items, choose “Internet Protocol Version 5 (TCP/IPv4)”,
and double-click it, or highlight it and click the “Properties” button below the list.
The DNS section is at the bottom. You need to choose the "Use the following DNS server addresses:"
and then click the “Advanced…” button to enter more than just one primary and secondary address.
In the Advanced Settings dialog, switch to the DNS tab.
Use the “Add…” button to add new addresses. And the Move Up and Move Down buttons to put the list in the order you wish. (First used will be a the top. Least at the bottom.)
And looking at my wired adapter’s list, I see that Comodo has gone and wiped out my manually entered list of 8 pairs (16) DNS server addresses, again!,… and inserted a pair of addresses I’ve never even seen before. I just have to figure out how to keep it’s updater from doing this, or I’ll switch to some other security product.
Anyway, you repeat the process for the other adapter if you use it as well. (Why we cannot just do it once is annoying.)