Well, something has got worse now. This morning I couldn't get 72W at all no matter what side I approached it on. So, even though it was below freezing, I went out and checked the dish. There was a visible amount of play in the motor tube. So, seeing as how the DG-280 has the backlash adjustment screw readily accessible, I tightened it up. But still no signal on 72W. Everything else to the west of south (I'm at about 77 degrees) is still spectacularly aligned, Hispasat is still there at its normal half-assed level, and I can even get one data transponder on 37.6. Just no 72! (61.5 is solidly behind a tree.)
I even tried bumping the position with DisEqC 1.2. (I use USALS for everything, since my motor alignment is good enough to do so.) No luck. No signal at all on 72. In fact, most of the time my strength reading is at 36%, which is Openbox-speak for "no signal", rather than 45%, which is Openbox-speak for "it looks like a transponder, but it won't lock".
Last night was the coldest night we've had this season so far, but if something had flexed in the motor mount or the J-tube, I wouldn't think I would still be having uniformly great alignment on the rest of the arc. I checked everything all the way over to 125 and it's still good, all the usual weak signals are coming in, Montana PBS, both SatMexes, the S2 on 103, even CBS Newspath. Everything back down through 83. That's a heck of a spread to be fine if something was that far off, I know, I've had poor alignment before.
The only thing I can think of is maybe the weight of the Invacom QPH-031 has something to do with it. Someday I should switch it back to a cheap plastic LNB again to see if it changes anything. I just don't like being out in the cold anymore. And I don't have a meter, so I'm definitely not hauling out receiver/monitor/extension cord til spring!