I can definately write a lot better than I can speak for I am able to take the time to think before I put the words down. And sometimes these very same words will come to me in a better sense than if I was sitting there trying to talk to someone. In my honest opinion I speak with my brain and I write with my heart, which is what makes all the differences.
The only other thing that I can think of that might tie up my tongue would be the classic call center training that I have to put up with each day. Due to the various stages of what we have to say changes so much I will start to say something then stop in confusion as to what I am suppose to be saying. It definately makes it even worse if for whatever reason the tongue decides that it doesn't want to be fluent so you sit there for about two minutes trying to say M&M or something that you don't have the smallest idea to pronounce while just seeming to stutter terribly until you start to laugh self-consciously at yourself before giving up.
Truly those are the most awkward times if you don't count the trying to talk and lead a phone call that you must concentrate on. In that case you are either going to not understand what you saying to the customer or you are going to definately write the wrong memos. Give me that old pen and piece of paper, maybe even a computer, any day for communications.