I know beforehand that i'm going to say a sentence that is complete & grammatically correct but out of context and can't stop it. what is this?

Expressive Language: You may have what is known as an "expressive language disorder." read about this here: http://en.Wikipedia.Org/wiki/expressive_language_disorder. Some of these are hereditary, with specific genetic causes, while others are more poorly understood. If you haven't already, you might want to consult a neurologist for a complete evaluation and therapy plan.