I haven't had this problem with the L920, but I did with various Android phones. I learned that part of it was that the screen creates a lot of heat when it's on, and uses a lot of GPU power. Too much heat messes with the functionality of a CPU or a GPU and can cause issues such as reboots. It's why so many computers have HUGE fans just to be stable.
It may be any number of things so, my question to you is, how long are you using your phone before it gets hot? What are you doing with it (editing lots of docs, playing games, making a video, etc)?