I'd second Aspienoid. I've found the exact same issue; I communicate far better through written word than verbal, but real-time online chat isn't much better for me than verbal.
What I find to be one of the most satisfyingly to-the-point explanations of what goes on in the Autistic brain vs. the NT brain came from a NOVA series titled "What makes us Human" I'm searching online for the specifics, but they escape me at the moment. Anyway...
In the segment, it was illustrated that Asperger's/Autism appears to include (if not completely stem from) a sort of impairment in the I-feel-what-you-feel part of the brain, which is where a lot of human-human communication takes place. Like a hearing impairment, this can range from slightly impaired (Asperger's) to profound deafness (Low functioning Autism). In the case of Asperger's especially, we tend to compensate by doing some of that communicating consciously, in the frontal lobe.
What I get from this is that a fair amount of communication that should be instinctive, unconscious and most importantly, instantaneous, is instead a conscious, and therefore more time-consuming, effort for us.
Therefore, in verbal conversations or real-time chat, we are struggling to keep up with the conversation in a way, whereas writing something out, we have a little more time to process what we are saying.
Just my educated guess though. I claim no expertise on the subject.
- Jo