Ipad 2 screen dims then brightens for no reason without touching

[tanialiya]

I am having the same or a similar problem - when I lay the iPad down while reading, say, to make a cuppa, it sort of dims and brightens all by itself and does not go into sleep mode a all which is very battery consuming. It seems to sleep ok on other screens like the home screen. It seems to sleep when I cover the tablet and there is a moment of black screen when I lift an edge...