Java game loop FPS problems

Question

I'm making games in Java. I'm using this game loop:

public void run() {

    this.requestFocus();

    double firstTime = 0;
    double lastTime = System.nanoTime() / 1000000000.0;
    double unprocessedTime = 0;
    double passedTime = 0;
    boolean render = false;
    double frameTime = 0;
    int frames = 0;
    int fps = 0;

    while (running) {

        render = false;

        firstTime = System.nanoTime() / 1000000000.0;
        passedTime = firstTime - lastTime;
        lastTime = firstTime;

        unprocessedTime += passedTime;
        frameTime += passedTime;

        while (unprocessedTime >= UPDATE_CAP) {
            unprocessedTime -= UPDATE_CAP;
            render = true;
            // TODO: update game
            if (frameTime >= 1.0) {

                fps = frames;
                frameTime = 0;
                frames = 0;
                System.out.println("FPS: " + fps);

            }
        }

        if (render) {

            frames++;
            tick();
            render();
        } else {

            try {
                Thread.sleep(1);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }

    stop();
}

I have a problem - FPS are dropping very low, even when there's only menu opened and pretty much nothing is happening.

FPS: 60
FPS: 61
FPS: 61
FPS: 61
FPS: 61
FPS: 48
FPS: 25
FPS: 8
FPS: 3
FPS: 4
FPS: 61

I have no idea why this is happening. If you want I can share more game code.

..................................

Answer 1

I know what happened: I was creating a lot of Font objects every tick:

public void render(Graphics g) {
   Font f1 = new Font("SansSherif", Font.BOLD, 20);
   Font f2 = new Font("SansSherif", Font.BOLD, 50);
   Font f3 = new Font("SansSherif", Font.BOLD, 135);
}

Now, I'm creating them only when game starts.