Odd input glitch

I have a script that toggles the drag of a physics object while x is held down and then when x is released it returns the drag to 0. The problem is that sometimes the key presses don't register, sometimes it wont register that the button is pressed and other times it won't recognize the key is released anyone see a problem with my code? Here is the related code:

 function FixedUpdate () {
 if(Input.GetKeyDown("x")){
 Brake(true);        
 }
 if(Input.GetKeyUp("x")){
 Brake(false);        
 }
 }
 
 function Brake (Down){
 if(Down){
 rigidbody.drag =  1;
 }
 else{
 rigidbody.drag =  0;
 }
 }