Same problem here. I've asked about it some time ago on here and got no where. At the time I didn't press the aspect of my set up that there is a machine connected via ethernet to the base station as well as a lap top connected to the base station via air, but that is the case with me and also both of you. Often I don't loose connection to the internet entirely but it becomes bogged down and flakey (often in connection with using P2P software). Other times, complete internet connection loss. Likewise turning base station and modem off and on usually solves (temporerily). I have had a few times where that doesn't allow it to work again, and disconnecting all wires from the modem then reconnecting again is necessary. Thing is there's so many things which could be to blame: ISP, the modem, the base station. How do we know (I don't although suspect) it is the base station?
I'm using the white round extreme b & g with internal dial up modem base station. Ethernet modem > base station then ethernet connection to mac, and air connection to mac.
My problem occurs when the mac connected via ethernet to the base station has been off all the time -- I was feeling this wasn't part of it.
There is a thread of some time ago which had a very similar problem (sans mac connected via ethernet though I think) and he found downgrading the firmware (a) solved the problem entirely (b) lost some security functionality of some sort.
Here's a link to the last of my posts in my previous thread which I asked about this problem in, which links to the above mentioned "some time ago" thread:
http://discussions.apple.com/message.jspa?messageID=4418341#4418341 (oh yeah just to save a possible bit of wasted time, about the first half of the thread that directly links to, apart from maybe the description of the problem itself, is a complete waste of time).
I haven't tried downgrading, maybe I will. The problem does seem less drastic now for me (which leads me to think maybe the problem was connected with my ISP as not a lot has changed at my end). The problem does still occur but more down to a dribble/flakey rather than outright stop.
Seems to me the whole thing's flakey and unlikely to work smoothly -- I've kind of given up / am resigned to that 😟
There's two things I should try but can't really face: taking the base station out of the set up and using internet > modem > laptop and seeing if the problem still occurs or not (it really is impracticle for me to do that though), and downgrading the firmware somehow.