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Pagerank is ephemera. It can evaporate overnight for many reasons.
Pagerank is by the page, not the domain = not domain wide.
Now, show me a domain that has some hefty inbound links from busy pages on the NASA.gov website or the NIH.gov website and we may have something to discuss. :)
(Please, unless it's NASA we're talking about we don't need to know the specifics about your source of Pagerank.)