I have recently taken upon myself a practice regimen: I go on chatroulette and try making conversation with random girls.

It has worked incredibly well so far - every girl that has stayed and talked to me, had an enjoyable conversation and we exchanged contacts. One even asked me to marry her (although jokingly, it did indicate to me how much she liked me).

The problem is that all those conversations were done via text-chatting (and video).

Whenever I text chat with a girl, it seems so easy - I can always quickly come up with witty and clever replies, and I come off as an intelligent person. The conversation just flows off my finger tips, and I often find myself talking too much. Since it also includes video - there might be some awkward silences here and there when nobody's typing - and I can handle those as well, but when speaking on skype later, it's a whole different story.

I suddenly feel very awkward and slow, and can barely get a full sentence out of my mouth, let alone come up with clever replies.

I feel almost as if I'm disappointing those poor girls who expected much more.

Why do you think this is happening? and how can I improve my audible communication skills?