How to permanently disable priority interruptions setting?

I'd like to revert to the pre-Android 5.0 Lollipop behaviour of having all notifications (including calls, messages and alerts) come through at all times. The problem is that whenever I configure the device to "always interrupt" after some time (usually in the order of hours) the device reverts to "priority" mode. This happens regardless whether I use the setting at Settings / Sound & notification / Interruptions / When calls and notifications arrive / Always Interrupt or Volume button down / ALL. How can I make the "always interrupt" setting stick permanently?