I may have found the problem. Adjustments with Google's "Smart Pricing" may be going on.
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How smart pricing works
We are constantly analyzing data across our network, and if our data shows that a click is less likely to turn into business results (e.g. online sale, registration, phone call, newsletter sign-up), we may reduce the price you pay for that click. You may notice a reduction in the cost of clicks from content sites.
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