I have FIOS also and sometimes I experience issues with Anandtech forums for some reason. Pretty much as you describe. Not sure if it's related or not but seems like nobody else complains about it when I'm experiencing the issues....Seems like it's possible to be ISP related somehow but makes no sense at the same time. Could be issue with one of the hops I guess.
Maybe next time you experience the issue try opening the command prompt and type the following and see what it shows.
Not sure if the following works or not. Looks to kick to Anandtech home page when clicking the hyperlink for some reason. ???
tracert
forums.anandtech.com and it'll trace the route showing the hops. For me there are a lot of hops along the way. Not sure if at times FIOS has an issue with one of them or not. Next time I experience the issue I'm gonna try it and see if it shows anything or not.