I have a Phillips TV (it’s about 2-3 years old) in the den. When you first turn it on, it will shut off after a few seconds. The screen goes black, but the “on” indicator light stays on. I have to turn it “off” and turn it on again. This may take 7-8 times to get the thing to stay on. Sometimes, after being on for several hours, it’ll shut “off” and I have to go through the routine again.
Any clues?
None here.
Could someone in your house have one of these?
Any new gadgets? New electronics? New neighbors?
Nope.
Dying batteries in the remote?
Screen saver? 
Unplug it from the AC power and let it sit unplugged for at least an hour. Plug it back in and see if this behavior continues.
Look at it this way:
Our smart assed answers are keeping the OP on the front page until someone who reallys knows his shit can come along and be of real help.
Nah. I’m not that altruistic. I did it just for the smart asseryness of it!
(Hope you get a real answer someday.)
Is it an “instant on” TV? If so it should show a picture immediately.
If it is not an “instant on” it will take time to warm up the electron gun.
My suggestions wasn’t a smart assed answer. The logic in modern TVs can get confused and stay in that state until power is pulled and they are left to sit a bit, then plugged in again. Not sure why this works, but occasionally it does. MBS’s problem sounds like a power supply issue of some kind that will require repair (or more likely replacement) to solve.
It is an instant on set.
Astro: I’ll try that.
Sorry, Astro. But look at our time stamps, won’t you? I obviously wasn’t meaning you.