I'm having the same problem, and because it is intermittent, it is particularly hard to debug.
I'm on a satellite connection through a D-Link WBR-2310 router (direct connected), running an up-to-date WinXP with Firefox 3.01. This morning I got the error message several times while trying to connect to
www.windstreampower.com/ and when I ran tracert for the same address, it timed out too (well, it just disappeared after the 11th segment...). When I tried the address in FF again, this time I got a connection but only some of the page displayed: no images, messed up format, etc. Pressing F5 got me back to the error again.
If I go to the same web site with IE7.0, I get something similar: either an error message or a partially displayed page. This has been happening more frequently in the past week or so, and on both systems here (the other is Vista with FF3.01 and IE7 as well). I'm stumped.