Diagnosis
The Auto Power Down feature is enabled. It's a default setting that shuts the soundbar off when no audio signal is detected for 15 minutes. A weak signal from your TV can trigger it.
How to Fix It
Disable Auto Power Down. Press and hold the 'Gear' button on the remote for 5 seconds. The display will show 'APD OFF'. Release the button. The soundbar will now stay on.
If That Didn't Work
The issue may be a loose HDMI connection causing signal drop. Unplug and firmly reseat the HDMI cable at both the soundbar and TV ends.
Heads Up
If you use optical audio instead of HDMI, check your TV's audio output settings. Set it to PCM, not Dolby Digital, if the dropouts continue with APD off.