This is an expression that will select random frames over a defined period, giving a "stutter" effect.
In this example, the animation has been set up to run normally for the first 1 second, stutter for .5 seconds, then resume running normally.
To make this work, you enable time remapping for the layer you want to stutter. Then you apply this expression to the time remapping property:
start = 1.0; //time to start random frames end = 1.5; //time to end random frames if(time >= start && time <= end) random(start,end) else time
Adjust the start and end times to suit your needs.