Mental Crohn's
In a situation when you must think of the right words for a sentence, but you fall under full hesitation. This is the higher form of a brain fart in to which your entire comprehension of the human language collapses just short of succumbing to a stroke.
"I was on stage to present my piece for the debate championship, but my brain just stopped. It didn't feel like a brain fart as much as full blown {Mental Crohn's."