I'm using a raycast detection to activate a boolean function in another script. This boolean then turns on or off a OnGUI screen. The problem that I'm having is when the state is true, the OnGUI flashes on/off like it's switching between true/false rapidly.

Is there a way that I can only call the function when the collision state changes, not on every frame?

From the collider script:

function Update()
{   
    var hit : RaycastHit;
    // check if we're colliding
    if(Physics.Raycast(transform.position, transform.forward, hit, speakarea))
    {
        // with a dog
        if(hit.collider.gameObject.tag == "dog")
        {
        otherScript = NPCObject;
        otherScript.ShowIntro();
        }
    }
    else{
        otherScript = NPCObject;
        otherScript.HideIntro();
        }
}

From the OnGUI script:

function ShowIntro()
{
showIntro = true;
}

function HideIntro()
{
showIntro = false;
}

function OnGUI()
{
    if (showIntro) {
            GUI.Box (Rect (Screen.width * .2 ,Screen.height * .2,240,360), "");}

}