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There is no way that the US advertisers want their ads to be visible outside the US.
My guess is that one of the AdSense servers doesn't have a working IP table and is just defaulting to US ads which is why we are seeing intermittant problems.
Adsense is broken or they are trying to cheat advertisers. I think it is the latter.
Better quit the network, then. You don't want to get arrested or sued as a co-conspirator. :-)