Well, I tried normalizing the motion but it didn't do much. I eventually just settled on the script selecting a random point and using a directional variable in motion = direction.normalized().

Here's the parts of the script that I've changed:

var destination = Vector2()

func _ready():
    _timer = Timer.new()
    add_child(_timer)

    _timer.connect("timeout", self, "_on_Timer_timeout")
    _timer.set_wait_time(randi() % 6+6)
    _timer.set_one_shot(false) # Make sure it loops
    _timer.start()

    randomize()
    destination = Vector2(self.get_global_position().x+100, self.get_global_position().y+100)
    pass

func _physics_process(delta):
    var direction = destination - self.position
    if speed <= MAX_SPEED:
        speed = speed + ACCELERATION
        motion = direction.normalized() * speed
    else:
        motion = direction.normalized() * MAX_SPEED
    if direction.x > 0:
        $AnimatedSprite.flip_h = true
    else:
        $AnimatedSprite.flip_h = false
    if speed < 10:
        $AnimatedSprite.speed_scale = .1
    else:
        $AnimatedSprite.speed_scale = 1
    move_and_slide(motion)
    pass

func _on_Timer_timeout():
    randomize()
    _timer.set_wait_time(randi() % 6+6)
    _timer.set_one_shot(false) # Make sure it loops
    _timer.start()
    destination = Vector2(self.get_global_position().x + randi() % 600-300, self.get_global_position().y + randi() % 600-300)
    speed = 0
    pass

But now what I'm trying to do is give my AI an Area2D that acts as the destination, and when the AI enters that area it sends a message to the AI's script to slow down (speed = speed - ACCELERATION). However, I'm having trouble sending a signal from the Area2D to the AI that it has entered that area.