Some Websites loading slowly-3 computers

OK, you've all seen this a million times. This is different.

Four days ago--suddenly--some web sites (AOL, Google, this forum) began loading extremely slowly--so slowly they often timed out. Or in the case of AOL, the first splash page appears immediately, but if you click "sign in" you might as well go watch an episode of your favorite TV program and then come back. And then sometimes the "regular" next screen will be there, and sometimes the bogus-looking secondary screen for slow computers. Then you sign in and go watch another program while it signs in. Then if you click "New mail" you need to go watch a movie and come back. Yes, it's that slow. In the case of this page, to navigate through 4-5 windows took about an hour and half.

But other web sites load just as fast as they always have--netflix, imdb, some banks (but not all!), yahoo, Amazon. And you can go from page to page in those sites with no issues. "Just as fast as they always have" = loads in less than a second; you click a link, it's there immediately, etc.

OK--here's the thing. This is happening on all browsers: Firefox, Safari, Chrome. It is happening on three different computers: a 2012 MacPro running 10.8.3; a G4 running 10.4.11 (these are networked with Ethernet); and a Macbook Pro running 10.8.3.

So yes, I've done all the usual things: turned off the Verizon actionnet modem for hours, turned it back on; shut down computer and turned it back on. Zapped the PRAM. Checked all the network settings. Checked the internet speed (DSL; speed checked twice at 2.92 mbps--normal).

Even though it doesn't seem specific to a particular browser, in Safari I emptied the cache, reset, etc. multiple times --no change.

I reset the modem to factory default and restarted--no change.

I've read hours and hours worth of stuff in forums--including this one, and of course some of the advice applies to obsolete systems, etc. and doesn't apply. This problem--in various forms--seems to be a constant one, going back 10 years+. But it always seems to be related to one computer or one browser. My problem is 3 different computers, 3 different browsers.

Plugs ins--maybe a problem for one browser on one computer, but not all browsers on all computers. Esp. the laptop, since it wasn't even connected to the others. And all the problems occurred on all the computers at the same time.

The only constant is the modem, since all the internet signals go through there. But I'm at a loss as to what the problem can be. When the sites stall, they say things like "waiting for response...." It seems to me that the modem must be slowing or blocking signals to some sites; why some and not others? Why did this start suddenly?

HELP!