Being a Detroit Lions fan, I see the same thing with them. They play great against similar or lesser teams, but against the great teams, they falter under the pressure.
I think the greatest difference is between the ears, rather than talent. They have to learn not to be psyched out by the pressure of playing the great teams, like the 49ers did today. They decided they wouldn't be psyched out, and physically punished the Saints. One of the most brutal games I've seen in years.
Next week, GB in a massacre.