Share your experience!
Just restart your phone regularly(1-2 days),the RAM memory will refresh.
It should be in power management,you can disable it there.But stamina mode does not use RAM.
That is an app you have installed.Stamina mode does not use RAM.What Android version do you have?
I also have JB on my Xperia P(4.1.2),no extended standby mode,just stamina mode and low battery mode.None of them you RAM memory.
I think you should consider a software repair with sony update service.