Probably the same issue as [webmasterworld.com ]
Look for clawbacks of clicks (clicks appear, then disappear 1 to 30 minutes later) which decrease over a few days, but leave you with a greatly reduced CTR which *might* slowly recover as your "punishment period" expires. Let us know if this matches. It seems like G has been rolling out a new formula that is killing selected publishers. They are blaming it on invalid clicks, which doesn't explain the recovery behavior if you have any traffic at all.