RPMs dropped on HIGH page views

It is standard for RPM to drop as your pageviews goes up. The reason is because Google Adsense displays high-paying ads first, which get clicked first. But after a lot of clicks on those ads, the high-paying advertisers' daily/weekly/monthly budgets get used up (they don't have infinite money to buy clicks with).

So once you use up all the high-value clicks, Google stops displaying those ads, and displays the next highest paying ad. And so on, and so forth.

So in other words, if you keep getting more and more traffic, your RPM will get lower and lower. But it isn't because the high-value clicks are disappearing, you are just using up all the daily allotment that those advertisers have paid for. Basically what you're doing is fully exploiting the maximum amount of earnings that you can get from those advertisers.

There's also the possibility that over time, some advertisers who paid well will end their advertising campaigns if they aren't making a good ROI, thus lowering your RPM by lowering the payout of the clicks. But that is a different issue and there isn't much you can do about that, unless you want to contact those advertisers directly and try to sell them direct ad space on your website, which isn't impossible to do but definitely takes more effort on your part to set up and maintain.