How to correctly calculate FPS in XNA?

I wrote a component to display current FPS.
The most important part of it is:

    public override void Update(GameTime gameTime)
    {
        elapseTime += (float)gameTime.ElapsedRealTime.TotalSeconds;
        frameCounter++;

        if (elapseTime > 1)
        {
            FPS = frameCounter;
            frameCounter = 0;
            elapseTime = 0;
        }
        base.Update(gameTime);
    }


    public override void Draw(GameTime gameTime)
    {
        spriteBatch.Begin();

        spriteBatch.DrawString(font, "FPS " + ((int)FPS).ToString(), position, color, 0, origin, scale, SpriteEffects.None, 0);

        spriteBatch.End();

        base.Draw(gameTime);
    }

In most cases it works ok, but recently I had a problem.
When I put following code into Update method of game strange thing starts to happen.

       if (threadPath == null || threadPath.ThreadState != ThreadState.Running)
        {
            ThreadStart ts = new ThreadStart(current.PathFinder.FindPaths);
            threadPath = new Thread(ts);
            threadPath.Priority = ThreadPriority.Highest;
            threadPath.Start();
        }

Main idea of this code is to run pathFinding algorithm in different thread all the time.

By strange things I mean that sometimes FPS drasticly decreases, this is obvious, but displayed FPS changes more often than once a second. If I understand this code FPS can't change more often than once a second.

Can someone explain me what's going on?

Edit 26.03.2010
I've posted also code of Draw method.

Edit 31.03.2010 Answers to Venesectrix questions
1) are you running with a fixed or variable time step?
IsFixedTimeStep and SynchronizeWithVerticalRetrace is set to true.
2)Were you moving the XNA window around when this occurred?
No
3)Did the XNA window have focus?
Ye