I have the same (or similar) problem with my n900. However I have experienced a gradual degradation in its wifi functionality. I purchased it November 2010, for an upcoming trip to the US. Here is how it has degraded:

1st two months - phone worked great, used it in the US (Im from Australia) and was able to download, torrent and practically everything (order flights, net banking) all at a reasonable speed. Wifi settings were default (powersave max and 10mw wifi).
2nd two months - whilst back in Australia, the powersave feature started to cause problems. The browser would hang after ~5-10 minutes internet browsing. Reconnecting would sometimes solve the problem, however on some connections it simply would not authenticate (public connection at my university). Turning powersave off solved the problem (intermediate didn't work properly either).
Last week - all wifi functionality has stopped working correctly (though gprs is still fine). The wifi icon shows its still connected, however no internet functionality - error "Problem loading page - Server not found".

To fix it I I have tried:
1) different combinations under the 'internet connection'
2) turned it on and off
3) deleted saved connections and reconfigured them (it sees the wifi accesspoint without a problem)
4) I will try reflashing my phone, however even if that works (some posts suggest it won't) it isn't an acceptable solution as it'll just occur again in 3-4 months.

PLEASE HELP! Anybody who is technically/software savvy, please give a definitive explanation or suggestion on how to permanently fix the wifi problem or even whether it can be fixed. I love this phone and since N9 is canned (N950 might also be) I fix is mandatory. I'm guessing its a software issue, however the degradation has been sort of linear so it may also be hardware slowly failing.