You should read my thread in Supporters about the headaches I was having pinging/loading certain sites. Think: Apple O/S, WinXPPro, Win7 vs./+ wireless router settings vs./+, vs./+ network security software (firewalls, etc.) vs./+ . . . 30+ hours, over the course of a week, to nail down and eliminate the issues. It came down to packet size issues and protocols used by different sites and router settings that "allowed" certain sites local access but blocked others, not by IP or DNS, but by how they delivered data. Argh . . and ARGH again. ;)
Sometimes there can be a local caching issue with an ISP that can delay "seeing" an update to DNS or viewing changes to a site. Rare, but it has happened to me and others.
With the growing complexity, capacity and utililization of that capacity of the Web - such as millions now streaming NetFlix - nothing surprises me less than hiccups of unexplained origin. I'm still running DSL, since I'm near to the local exchange, but when it rains - due to the age of the cables - I will sometimes suffer a degradation.
Throttling? Blocking? Never heard anyone, at any ISP, admit to it. Of course, what does the guy/gal on the phone in the helpcenter know except what their 2 days of training told them to say? :P