Your connection is low (or has a big latency), may be a dial-up. Another version is that you installed Mibew on a slow virtual server.
Go to "Settings -> Performance".
Set "Operator's console refresh time" = 4 (or more),
"Chat refresh time" = 4 (or more).
Changing timeouts (only if it still tries to reconnect):
Edit /webim/js/source/chat.js, line 62: this._options.timeout = 5000; // 5sec, value in milliseconds
Edit /webim/js/source/users.js, line 9: this._options.timeout = 5000;
Minify Javascript (you'll need Java and Ant (ant.apache.org)):
in /webim/js/source/ run "ant"
or copy all .js files from /js/source/ -> /js/162/