PSP locks up on certain games.?

So I bought crisis core for my PSP but when the game starts my PSP locks up. It will still be running but my ons won't work except for my ons at the bottom. This is not the first game that this is had happened to. It does the same thing to the legends of war: patton's C. Demo and the Valkyria Chronicles 2 demo. If anyone know how to fix this problem please tell me.