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I realize there are several variables at work here, but the sites are reputable (PR4 on Google) with a growing number of relevant inbound links and pages/articles, and used to rank pretty well, so we're scratching our heads a bit...
Has anyone done any sort of analysis on how .US sites have fared in the SERPs over the past year or so? Any anecdotal evidence out there of an algorithmic "penalization" of .US domains?
Add a bit more weight if the cc TLD is hosted in the appropriate country.
Add a bit more weight if the searcher chooses to limit their search results to a specific country.
Otherwise, your mileage may vary according to dozens of other variables, weightings, filters, etc.