DIAGNOSIS This is the Auto Standby feature. It's designed to save power when no audio signal is detected for 20 minutes. It is enabled by default.
FIX To disable it, press and hold the 'Bluetooth' button on the remote for about 5 seconds until you hear a tone and the status LED blinks. This turns Auto Standby off. You'll need to repeat this after a full power cycle to turn it back on.
IF THAT DIDN'T WORK If it still powers down, the audio signal from your TV may be dropping out. Check your TV's audio output settings for a 'power saving' or 'audio pass-through' mode that might be interrupting the signal.
HEADS UP With Auto Standby off, the soundbar will remain in a low-power listening mode indefinitely, which uses slightly more electricity. Plug it into a switched outlet if you want manual control.