"Have you had any problems with that happening before?" the Doctor asked. "We had---at least, I think, we had it happen before. And the storms. All the time."

He reached a hill and struggled up the side, nearly slipping on the sand. "But not when we were apart. Just when we were together."

But there were still big holes in his memory. Maybe they'd already uncovered the answer to it and the Doctor had missed it.