Uhm.. Please let me think for another 10 seconds....
EDIT::
Change the last function to:[
code:
var Index = new Array();
function RandomMessage() {
      Index[0] = Math.random().charAt(4); // could be any number between 2 and 18
      Index[1] = Math.random().charAt(7); //  // could be any number between 2 and 18
      Index[2] = Math.random().substr(Index[0],Index[1]); // Please note!! You need 99 messages, or the chat will send "undefined";
      return RandomMessages[Index[2]];
}