Well the "it fixed itself phase" was short lived. The set top box started exhibiting the same behaviour again. I just keep pressing the power button (either on the remote or on the front of the box) repeatedly until it stays on. Takes about 20 button presses before it holds and stays on. So it seems like some poor joint somewhere that is getting intermittent contact, but I took it apart again and looked at all 3 boards on both sides, and nothing stands out, I don't see any cracked solder joint etc. But looking at all the different sizes of capacitors on all the boards, getting replacements...

I watch some basic TV using an ATSC tuner set top box which outputs HDMI to my LCD TV. The brand is Centronics, which seems like a fairly low cost (ie cheaply made) product. The UI is a bit crude looking compared to what I've seen on TVs that have the ATSC tuner built in (but it works!). Anyway, the set top box has always had an odd behaviour, which is that turning the device on using the remote would sometimes not "stick", that is, it would turn on momentarily and then immediately turn off. It didn't do this all the time, but it did this randomly since I've had it, which is 2009. Recently...

Mech eng background, and mostly have tinkered with car repairs/maintenance, but recently have ventured into fixing electronics, mostly to keep them going. One was an old, free-to-me LCD computer display. I used it for several years, and suddenly it wouldn't turn on. Replaced a bad capacitor on the power board, and it was back in business. The other was a garage door opener. Again, replaced a capacitor in the power section of the board and got the door opener working again.