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.

I just Do this and it works…

float smooth;
float smooth1;

void Start () {

    smooth = PlaneMovement.GameSpeed;
    smooth1 = PlaneMovement.GameSpeed;
}

PlaneMovement.GameSpeed = Mathf.Lerp (PlaneMovement.GameSpeed,smooth,Time.deltaTime);
    if(SpeedUp){
        SpeedUpTimer -= Time.deltaTime;
        if(SpeedUpTimer <= 0){
            SpeedUp = false;
            smooth = smooth1;
        }