Check out:
i had a somehow rather similar problem in the past:
+Internet didnt work on my PC, while it did work on another
+while it worked on my machine, it didnt on another
+sometimes both worked
+sometimes none
Btw: router showed perfect dsl connection state, internet was perfectly connected from what my pc said (connected with 100Mbts, and so one...)
-automatic ip assignment sometimes didnt work
-connection graph showed NO input (download) signal
-all internet applications using http or html protocols wont work
-sometimes programs using udp connections still worked, eg. sa-mp or TS
-so it was kinda weird, sometimes i had to tell ppl on samp, that i cant talk on xfire, because my internet doesnt work ._.
If you can find any of these points in your problem let me know,
maybe we could search for a good solution together...
oh btw i recently solved that kinda (lol), cuz i phoned my isp and complained that i will change my provider if this keeps happening, since then it worked pretty good XD
oh and i asked some fella from my university, and his opinion was that some broken electronic device somewhere in my house or at my neighbour's cold cause interferrence. well -.- i dont really know if i should believe that XD