Attention writers, help me out!

How do you write a character who isn't particularly articulate? Specifically I'm trying to write Charlie from Numb3rs, who when nervous tends to say "Um, well, it, well, um" before getting to whatever he actually means. I want to show that verbal hesitation/fumbling because it's crucial to the characterization of that moment. But I don't want to make my dialogue clunky or boring. I've tried using shorthand--don't try to SHOW the fumbling just say something like Charlie fumbled with his words for a moment as he tried to speak but that is not totally satisfactory either.

Has anyone else thought about this sort of thing? I'm sure there are other characters like this. What do you do? I'm sure that answers to this problem are going to be highly situational depending on who is being portrayed and the style in which it is being written, but I'm interested in some insight.