Timer doesn't work when another button is pressed (applet)

Question

I'm making an applet which has two buttons: step back and step forward. I have the same timer in every button to execute an animation. If I push step forward it works fine, and the animation runs, but if i push step back, the animation doesn't run or is bad runned (in a wrong position and velocity). I guess the problem is the timer doesn't stop correctly and is running when start the timer again, but I don´t know how to solve it.

This is the code of the step forward button:

//Code of the button "Paso a Paso"
        this.botonPasoAPaso = new JButton("Paso a paso");
            this.botonPasoAPaso.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent arg0) {

            //Avoid timer to accelerate 
            if (timer != null && timer.isRunning()) {
                timer.stop();
            }                           
            //Code of the timer, makes an animation 
            timer = new Timer(35, new ActionListener() {
                public void actionPerformed(ActionEvent e) {
                    if (pasoAPaso <= listaPaquetes.size()) {
                        Paquete p = listaPaquetes.get(pasoAPaso);
                        p.animar();
                        panelGrafo.removeAll();
                        panelGrafo.updateUI();
                        panelGrafo.setPaquete(p);
                        panelGrafo.setAnimar(true);
                        panelGrafo.repaint();                           
                    }                       
                }
            });

            timer.start();  

            pasoAPaso++;                
        }           
    });

This is the code of the step back button:

    this.botonAtras.addActionListener(new ActionListener() {
         public void actionPerformed(ActionEvent arg0) {
            if (timer != null && timer.isRunning()) {
                timer.stop();
            }


            timer = new Timer(35, new ActionListener() {
                public void actionPerformed(ActionEvent e) {

                    if (pasoAPaso < listaPaquetes.size() && pasoAPaso>=0) {

                        Paquete p = listaPaquetes.get(pasoAPaso);
                        p.animar();
                        panelGrafo.removeAll();
                        panelGrafo.updateUI();
                        panelGrafo.setPaquete(p);
                        panelGrafo.setPasoAPaso(pasoAPaso);
                        panelGrafo.setAnimar(true);
                        panelGrafo.repaint();
                        //Actualizar matriz del render para pintar celdas
                        RenderTabla.matrizTotal = new Cuadrado[pasos.get(pasoAPaso).length][pasos.get(pasoAPaso)[0].length];
                        RenderTabla.matrizTotal = pasos.get(pasoAPaso);
                        tabla.introducirDatos(pasos.get(pasoAPaso),false);                          
                    }                       
                }
            });

            timer.start();                  

            pasoAPaso--;            

        }                   
    });

This is a video of the applet to see what's happening If you see the video, when I click the "Paso a Paso" (Step forward) button, a truck is animated, but when I click the "rewind" (Step back) button, it doesn't work (appears a truck but is not correctly animated), and if i click again nothing happens.

Thanks.