The symptom you describe is caused by the TV's power supply shutting down. This can be obviously be caused by a number of things, but the prime suspect and most likely reason is a well known fault that develops in that range of chassis centered around the vertical scan IC, and being either bad solder joints on the IC / a capacitor failing (C926) / or indeed by the main component itself failing. These components are relatively inexpensive, however that only applies if you know someone with the ability to fit them for you.