If you have it paused or resting at a menu screen for too long, its auto-shutoff feature might be activating. If this is the case, you should be able to fix it by find the setting somewhere in your menus and turning the auto-shutoff (or whatever they might call it) option off.
If it's turning off while you're playing a tape or disc, then my guess would be that there's something wrong with the programming or circuitry, but that is definitely a GUESS.
Josh
November 2006