How the completed file works:

File #1: Generate Random Timing with Timer

Make a movieclip instance move horizontally in a random increment for a random length of time, then move in a different random increment for a different random length of time. Keep repeating it.When it hits the stage edge, it reverse the direction.

Extra Effort #1:

Extra #1: Generate Random Timing with Timer

Make a movieclip instance move at a random direction in a random increment for a random time, then move at another random direction in a different random increment for a different random time. Keep repeating it.