Another thought is what are you doing at the time of day when there is a sudden drop? Is is at a consistent time?
For instance, in the morning I stream music via bluetooth on my way into work. At work I usually text and use data regularly and data drops fairly consistently but at the end of the day I usually call my wife and talk to her via the handsfree BT in my car during my 45 minute ride home. I usually leave work with around 75% battery but by the time I get home, it has dropped to around 50%.
Just a thought...