No stable picture, due to power surge.
You have an impending power supply failure.
I would not leave the set on and unattended. There are mulitple protective measures against bursting into flame but I trust none of them completely.
I just answered a few other complaints similar to yours and pasted some of my response below-
I'm afraid you are right suspecting the power supply; most modern sets use switching power supplies and they often fail when the source AC is screwy.
It might not have been the low-normal-low-normal swings but an unseen, short term surge that killed it.
The cycling is occurring to prevent a potential fire or more severe damage; a component has been whacked and the supply is detecting a fault condition, probably excessive current from some circuit.
I'm afraid you are stuck with a repair, if it was mine, I'd look inside first but some basic instruments are needed and technical background.
I highly recommend to homeowners that they install a 'whole house" surge protector. Especially in lightning prone areas.
With installation, around $200-300 US although an intelligent person can do it, it will require shutting down the home.
The surge protectors range from ~$110-200; most have a long warranty and many include insurance to cover items damaged despite the protection.