Adsense Unit Impressions/Page Impressions Problem

I have a couple of hundred sites with Adsense. Per Google Adsense Reports from the 14th to the 24th page impressions halved, then from 25th to 28th came back again. Now have halved again. Over this time Google Analytics and traffic per the server has stayed relatively steady. Google Adsense stats show coverage to be steady. Example is [login to view URL] which currently shows in Adsense Reports about 10% of the page views I would expect. Some ads are sents via OpenX, some are not. Identify the cause of the problem, fix.