That is not normal. Please check ventilation and apps installed.
Notes: (normal use)
-If you have screen saver enable the device goes to sleep after some time of inactivity. (Defined by the user)
-if the device is working in normal conditions and the hardware is good the box should work fine
-the tests that i made with this device: (official firmware V.001)
never crashed and never shutdown for itself.
Please tell us more details :
-box shutdowns completly or goes to sleep?
-Are you using official firmware or custom? (Slimbox OS is available for this version now)
-What apps are you using?