To keep users from "flooding" other users with message that might cause a DoS-like "attack" on the receiver's client, most IM protocols use a kind of rate limit that'll only allow a certain amount of information to transfer from one person to another in a given amount of time.
If you're hitting the rate limit, try not to type so fast
Rate limiting, which prevents a user from sending too many messages in a short amount of time. Once a user is rate limited, they are temporarily unable to send messages, but may still receive messages.
You're either typing too fast (sending a lot of messages in a small amount of time) or you're running an ancient version of Adium which doesn't properly space out its queries about user away messages. Make sure you have the latest version. If that's not the problem, learn how to type in compete thoughts instead of splitting them up amongst 8 lines sent in rapid succession.