This should do the trick ????
choices = "qwertuaslh";
str = "this is my string";
tStart = 1; //time at which the animation starts
tStop = 2.4; //time at which the animation stop
seedRandom(index, true);
charResult = [];
s = "";
// assign each characther a random stop time between tStart and tStop
for(i = 0; i < str.length; i++){
num = random(tStart, tStop);
t = Math.round(num * 100) / 100;
charResult[i] = t;
}
if (time < tStop){
for (i = 0; i < str.length; i++){
if(str[i] == ' ') {
s += ' ';
} else {
if(time >= charResult[i]){
s += str[i];
} else {
idx1 = Math.floor(random(choices.length));
s += choices[idx1];
}
}
}
}else{
s = str;
}
s