Don't shoot the messenger, but posting CTRs, clicks and impressions is prohibited.
See Section 8 of AdSense Terms and Condtions at https://www.google.com/adsense/terms.
"You agree not to disclose...click-through rates or other statistics relating to Site performance in the Program provided to You by Google"
That said, I don't know what your CTR was before the period you pasted in, but be aware that impressions includes all unpaid PSAs. In other words, if your pages are viewed 1,000 times and 95% of the page views show PSAs you might expect to see 50 impressions ( (100% - 95%) * 1,000 ) in the AdSense reports, but it shows 1,000. This is not a guess, this has been confirmed by plenty of my own testing which I've shared in other threads and which others have confirmed.
Since clicks of PSAs 1. result in no payment and 2. do not count towards clicks which are displayed in the reports, the first thing I'd look at whenever CTR goes way down is whether there has been a spike in pages showing PSAs.
The only effective way to do that is to implement Google's fairly new Alternate Ad parameter - see https://www.google.com/adsense/faq#basics10. If the alternate ad is hosted on a page on your server, you will be able to track how many times it's accessed (assuming you have raw log access). You'll also be able to track which pages it's being called from. With that information you'll be able to determine, for example, that Page 1 was viewed 10 times and showed 0 alternate ads, Page 2 was viewed 10 times and showed 3 alternate adds and Page 3 was viewed 10 times and showed 10 targeted ads. Based on that, you might conclude Page 1 consistently shows content-targeted ads, Page 2 may have less supply of ads than demand or users from certain countries may not see content-targeted ads and Page 3 may never show content-targeted ads, possibly because there is not enough content for the AdSense to determine what the page is about, the page contains "negative content", Google can't spider the page, etc.