DIAGNOSIS: The TV's 'Auto Source Change' or 'Input Signal Plus' feature is being triggered by a flicker or standby signal from a connected device, like a cable box or game console that's on but asleep.
FIX: Press the home button, go to Settings > General > External Device Manager > Input Signal Plus. Turn it OFF for the HDMI port your problem device is using (like HDMI 1). Then, go back to External Device Manager and select 'Input Settings.' Turn OFF 'Auto Source Change.' This stops the TV from constantly scanning.
IF THAT DIDN'T WORK: The culprit is likely the connected device itself. For a cable/satellite box or gaming console, go into that device's own settings and look for any 'CEC' or 'Device Control' options (like Sony's Bravia Sync, LG's Simplink, etc.) and disable them. This prevents the device from sending wake-up signals.
HEADS UP: If you're using an HDMI switch or splitter, they are notorious for causing this. Connect the device directly to the TV to test. Also, cheap or damaged HDMI cables can leak weird signals that trigger this.