Wilson, Timothy D. & Schooler, Jonathan W. (1991). Thinking too much: Introspection can reduce the quality of preferences and decisions. Journal of Personality and Social Psychology, 60(2), 181-192. [PDF link]
Nice little experiment that shows that giving a reason for your decision or prediction leads to judgments that are less congruent with expert decisions than controls.
Moral of the story: sometimes you gotta go with your gut.