black screen, white screen are both signs of low memory, multitasking and ICS management issues other have spoken about in this thread and it mainly falls under multitasking.
clearing cache and remove temp files, etc will help the OS. also check your downloads and if you have a lot of downloaded files, these will also add up.
the white and black screens you see are from the OS loading apps into main memory. make sure you close out "recent apps" as well.