Who's stealing my battery?

My Galaxy S2 Plus always had a problem with battery consumption.

This is a normal daytime battery usage:

Then, sometimes out of the blue, battery usage increases drastically:

I still haven't found out the root cause.

I even upgraded to Android 4.2.2, with no luck: the issue still persists.

Here is the usage breakdown: what's causing Android to use more juice than the phone and the screen, usually the most power hungry subsystems?

I used CPU Spy, but it doesn't seem to be an issue of deep sleep.