One advantage you have in XP is that you can check the event logs to try to get a handle on what's happening.
At the times when you cannot access the internet, are your machines able to see each other on your local network ?
I'm guessing you are on cable, so are you using the mac address cloning feature ?
I'd try taking your primary box offline. Just unplug the damn thing.
The fact that your BSD box can stay online should be enough to prove that it's not a router/ISP thing.
Interesting problem, I'm curious to know your results.
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Oh, and I'd also make sure I had the most recent firmware on the router too. Just a thought
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