Major championship pressure dude. The best book written on the subject:
"The Inner Game of Tennis"...OK, it's about tennis. But it's absolutely applicable to golf.
Read it dude.
In a nutshell:
http://www.youtube.com/watch?v=SUdTxXkecr8
You need to "Get in the Zone" as the pros says. Or "Be in the State of Flow" as the IGoT states...