Mysterious Facebook ads CTR drop! - please help

Hi,

I created a page post ad, and selected the newsfeed and right sidebar placements.

I then selected a highly targeted audience of 7 million people.

The first 3-4 days, the Click Through Rate was high (about 3.5) and the Cost Per Click was low, about $0.18.

However, after about the 5th day of running the ad, the CTR suddenly plummeted to about 1.5 and the CPC started rising to $0.25.

My question is, why did this happen?

I don't see how it could be audience saturation, because the actual campaign reach was only about 350K - nowhere near the 7 million potential reach.

And clearly my ad creative is fine, because it was working well previously.

So what caused the CTR to drop and the CPC to increase so suddenly and dramatically?

This is not the first time this has happened to me. This is a pattern I have observed with all my Facebook ads:

1) I select a large, highly targeted potential audience, and the first 3-4 days are great; high CTR and low CPC

2) Then suddenly, for no apparent reason, CTR drops and CPC rises, even though the ad has reached nowhere near its potential saturation point.

I would be very, very grateful for any help in this matter!

Cheers