Switzer is the most obvious one I can think of, his last 4 years OU finished 3, 3, 3, 14 and went 42-6. Maybe not quite his peak, but Tressel isn't at his peak either. There must've been others, just can't think of them right now.
thanks I didn't know about Switzer's departure.
In PC's case, I was thinking of how the bottom fell out from under the team in late 2009.