I have a lot of trouble talking out loud, at least in feeling I'm coherent.
If it is something I have thought about a great deal, I can sound very articulate. But often, especially in "personal" discussions, I'm thinking and exploring as I go, and it's very hard to find the exact words I want to use -- it's more like sketching, where I start by speaking very very generally, then slowly hone in as I go, dropping the false turns and clarifying the parts that are right.
But I hear how bad I sound sometimes (bad = VAGUE, very vague!) and so do others, and they've learned to be patient with me and give me some more talking time so I can figure out what I want to convey.
I notice that when I talk to or write to ENTPs, they tend to bowl me over. Very wordy... and go on and on, they've got words galore. I tend to be more precise, or at least pick over my words a lot more (especially when writing). It's sort of amusing, in a good-natured way -- I laugh to myself sometimes when I get their letters, they just ... talk and talk! No shortage of words.
A last point: When I do get honed in on an idea (or as I hone the idea), words MATTER. Maybe I am sketching at first, but at some point I choose certain words because they capture the nuance of what I mean. I'm being very careful. And I've been highly annoyed or frustrated when someone either responds to me without considering the nuances (and sometimes even changing the words, without realizing they've now changed my meaning) or doesn't realize how very very purposefully precise I was being.
The need for precision is another reason I struggle over words.