Thanks for the replies!
schan1269: what do you mean about the speakers being "woefully inefficient" at 86 db? What does that mean? Why would a "bargain receiver" have a problem with them? They're bargain speakers, after all. $250 a pair a the Paradigm dealer, as I recall. They've always sounded wonderful to me, whether I drove them with a Pioneer SX-1250 (a 70 pound vintage 160 watt per channel ultra well made monster), a little NAD 3020, the Onkyo TX-SR605, or the new Sony STR-DH810. I'd have to turn the NAD up pretty high to get them loud, but I once drove them all night for a dance party (ie, with *plenty* of volume) and I never noticed a problem.
MichaelJHuman: Thanks for the suggestion!
I disconnected the speakers and left the unit on 12+ hours: No power down.
With Left channel on a cable music station for 12+ hours: No power down.
Cable music plus Right channel, 12+ hours: No power down.
Music from media center PC via HDMI, Left channel: Powered off in about 2 hours!
I know for a fact that it shut off while I was browsing these forums the other night - it happened twice in about two hours with both channels hooked up. My roommate swears it kicked out while watching a Netflix movie on the PC, too.
So now that I've written notes, I must say I think I was mistaken about the Sony turning off regardless of the input. The only times it has powered down that I'm sure of have been while using the PC.
This is progress!
I do not have the screen saver active on the Windows 7 pc, and I turned off the monitor power down feature since I noticed that when the TV turned off, the Sony would turn off, too. And I know that when I was browsing the other day, I couldn't have let it sit still more than a minute or two. I was actually mid-sentence when it kicked off, so I think we can rule out a screen saver problem.
Any ideas at this point? I'll go looking for more specific solutions...