Ooooh, a data point:

In every case of a "signal lock drop" I've had, if I go back and play the specific area again, *but only start my DVD player within about 5 - 10s of the offending portion*, no signal drop. If I go back to about 60s before the dropped part, bingo, dropped again (very repeatable). So if it was a mastering problem, seems to me it should be dropped no matter how far in advance of the "bad" part I started my player?

I presume that in the 5-10s case, the 950 is still in "looking for a signal" mode, so even if there's too much of a gap, it maintains the lock. But 60s in advance, the 950 goes back to "normal lock" mode, and hence the dropped lock. ??