I had my cable modem go out in a similar way...of course it took 2 hours on the phone to convince the guy it wasn't MS Office creating the problem :-) (that was actually true)
It would only stay working if I put it right in front of the air conditioner to keep it cool.
Best thing would be try another computer with that connection, but if that isn't available, the next thing comes to mind would be a lot of connections. Try NETSTAT command in command prompt when you get the slowdown and make sure there isn't a ton of connections hitting your computer (from spyware, viruses). This happened at an office computer I was to fix. There were so many connections, it couldnt get anywhere.
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