Push button for Firgelli linear actuator

I’m using a Firgelli micro linear actuator (http://learn.robotgeek.com/demo-code/119-arduino-linear-actuator-tutorial-button-control-of-a-small-linear-actuator.html). I’m trying to get my code so that when you press a button the actuator will increase to a set length and stay there until the button is pressed again. Right now, I can get the button to register an action but once in a while the actuator will just activate by itself, moving up and down with no input. I’m not sure why it’s doing this.

//Includes
#include <Servo.h> 

//Defines

#define LINEAR50PIN 9        //Linear Actuator Digital Pin

#define KNOB_PIN 0       //knob Analog Pin



//max/min pulse values in microseconds for the linear actuators

#define LINEAR50_MIN  1800     //
#define LINEAR50_MAX  1200    //


Servo LINEAR50;  // create servo objects to control the linear actuators
int knobValue;  //variables to hold the last reading from the analog pins


int linear50Value = 180;  //current positional value being sent to the linear actuator. 

int speed = 10;

void setup() 
{ 
  //initialize servos
 
  LINEAR50.attach(LINEAR50PIN, LINEAR50_MIN, LINEAR50_MAX);  // attaches/activates the linear actuator as a servo object 

  //Analog pins do not need to be initialized
  
  //use the writeMicroseconds to set the linear actuators to their default positions

  LINEAR50.writeMicroseconds(linear50Value);
} 

void loop() 
{ 

  /**************linear actuator Positions *******************************/
  //read the values from the analog sensors
  knobValue = analogRead(KNOB_PIN);


 
  linear50Value = map(knobValue, 0, 1023, LINEAR50_MAX, LINEAR50_MIN); //Map analog value from the sensor to the linear actuator
 
  //use the writeMicroseconds to set the servos to their new positions

  LINEAR50.writeMicroseconds(linear50Value);
}

Found out I needed a pull down resistor. http://arduino.stackexchange.com/questions/8898/why-does-my-servo-twitch-and-turn-on-its-own