why my game is not getting its normal speed back with this code?

 void OnMouseDown () {
         if(name == "SlowButton"){
             PlaneMovement.GameSpeed = 0.5f;
         }
         if(name == "MediumButton"){
             PlaneMovement.GameSpeed = 1f;
         }
         if(name == "FastButton"){
             PlaneMovement.GameSpeed = 2f;
         }
         if(name == "VeryFastButton"){
             PlaneMovement.GameSpeed = 3f;
         }
         Application.LoadLevel("game");
         }

I enter with this code into the Next Scene and there i have a code for some extra Power like speedup while collision occurs for that the code is:

 if(C.tag == "Dimond"){
             Camera.main.GetComponent<StoneInitiate>().DimondList.Remove(C.gameObject);
             Destroy (C.gameObject);
             Camera.main.GetComponent<StoneInitiate>().StoneCounter+=20;
             smooth = 4f;
             SpeedUpTimer = 5f;
             SpeedUp = true;
             }

speedup code is:

 PlaneMovement.GameSpeed = Mathf.Lerp (PlaneMovement.GameSpeed,smooth,Time.time);
         if(SpeedUp){
             SpeedUpTimer -= Time.deltaTime;
             print(SpeedUpTimer);
             if(SpeedUpTimer <= 0){
                 SpeedUp = false;
                 smooth = PlaneMovement.GameSpeed;
             }

It Works Fine when collision occurs but smooth takes value 4f and It Doesn't get value form

PlaneMovement.GameSpeed

please help me for that I am new in Unity. Thanks in advance.