This is a real quick idea, you'll want to play with the numbers, but you should get the ideas. Attach to a DirectionalLight I would think.

var minTime = .5;
var thresh = .5;

private var lastTime = 0;
private var myLight;

function Start()
{
    myLight = GetComponent(Light);
}

function Update ()
{

    if ((Time.time - lastTime) > minTime)
        if (Random.value > thresh)
            light.enabled = true;
        else
            light.enabled = false;
    lastTime = Time.time;
}

thresh is the minimum threshold between 0 and 1 that a random number must exceed to turn the light on. minTime is the minimum time interval to check that random number.

A more realistic effect might have a 'strike check' time (between 'strikes') and each strike would be 'on' for some small amount of time, during which rapid flashing would take place. I'll leave that to you.