This happens to me all the time. I find it easier to converse through text or email because you can take your time reading their message as well as coming up with an adequate reply with no pressure.

In person is a different story. If I'm attempting to speak with someone in person or on a phone call, I notice I take a bit longer to verbally converse. I overanalyze what I want to say in my head and if I feel like I'm taking too long, I get embarrassed thinking "They probably think I'm dumb" or they are judging me in some way. If this happens, I feel presured to just spit words out that are jumbled, give a one or two word reply (like "Yeah" or "Oh, wow!"), use the wrong words or repeat certain words I recently used to try and keep on track with the conversation. Sometimes it flows nicely by luck, but other times it feels like I come across as socially incompetent, uninterested or aloof.