Thanks for your responses. It's reassuring, though disappointing, to find that I'm not the cause of the problem. I guess I'll just have to calibrate the cameras more often.
Still, I should take Bernard's advice about treating both of them with more respect. I'll stop telling the D-700 that it'll never be clever enough to get into a really good college, even though, deep down, I know it's true. As Tolstoy probably never said, "Happy camera bags are all alike; every unhappy camera bag is unhappy in its own way."
Thanks again. MB