Look at your powersaving settings. My PS3 is set to turn off after two hours of inactivity. That way, I can fire up a movie on Netflix and then go to bed. If I fall asleep before the movie turns off, the system will shut itself down and my TV turns off if the device connected to it turns off.

That may be the culprit. Check that out and come back to me. We'll go from there.

Well. Nevermind, then!

Glad to see your problem sorted. However, you can get around that problem (having a small inactivity setting) when downloading large games - try to turn the power off onscreen and this will bring the "turn off ps3 after downloading" option on screen .

lol after the second sentence I knew exactly what it was. just changed those settings myself a couple of days ago.

Since Your PS3 is old and runs with the hotter 90nm fabbed Cell and RSX chips, and assuming you haven't maintenanced it yet......

I suggest you take your PS3 apart and change the thermal paste.  Use ARCTIC SILVER 5 or some other good paste.  You can do it yourself with youtube tutorials or take it to a local shop in your state (I assume you live in the US).  I have a Fat 80 and I did mine myself.

Launch PS3s are notorious for dying, so, I wouldn't procrastinate. 

not to mention a god cleaning will do wonders. You'll be amaized at how much dust collects inside your ps3. When i opened my 40gig up there was huge dust ball collection inside. too bad the effort was for naught..i bought some arctic silver thermal paste for cheaps, wouldnt you know it, i got what i paid for :(