Why is my java made pacMan clone sometimes laggy?

Question

I'm working on a pacman clone in java using eclipse and sometimes it appears laggy more specifically the movement of pacman/ghosts is slow, sometimes its fine. Once it has happened while I was running it so it wasn't after I added code and it doesn't seem to be after any specific event in game. I can't find any trigger or produce the lag on purpose

The resource manager shows the same cpu usage(only around 50%)/memory usage . Aswell the FPS seems to be around 200 consistently through lag and during the periods where it is working well.

Does anyone know what this could be?

Is there any information I left out that could be of use?

edit - I am basing movement on a timer is that bad? I will post the movement relevant code below is there a good way of posting the whole code on here?

Timer movement = new Timer(20, new ActionListener()//500 is time in milliseconds between
      //calls to actionPerformed as far as I know.
    {
        public void actionPerformed(ActionEvent arg0) 
        {
            if(movingUp == true)
            {
                moveUp();
            }
            else if(movingDown == true)
            {
                moveDown();
            }
            else if(movingRight == true)
            {
                moveRight();
            }
            else if(movingLeft == true)
            {
                moveLeft();
            }

        }

    });


public void moveUp()
    {
        yPos -= 1;
        this.rect.y -= 1;
    }

public void setDirUp()
    {
        movingUp = true;
        movingDown = false;
        movingRight = false;
        movingLeft = false;
    }

in the main class in public void keyPressed:

if(keyCode == KeyEvent.VK_W)
        {
            if(pacMan.isUpHittingWall == false)
            {
                pacMan.setDirUp();

                pacMan.isDownHittingWall = false;
                pacMan.isRightHittingWall = false;
                pacMan.isLeftHittingWall = false;
            }

        }

edit 2 -Thanks for the help guys. I have the movement using System time now and it seems to have fixed the issue because I implemented it only for pacman at first and the ghosts were still slow. Now there is an issue where moving right and down are much slower than moving left or up The only difference I see is that right and down are both adding and left and up are subtracting. What can I do about this?

The updated code is below.

//updated movement code
public void moveUp(long timePassed)
    {
        yPos -= vy * timePassed;
        this.rect.y -= vy * timePassed;
    }

    public void moveDown(long timePassed)
    {
        yPos += vy * timePassed;
        this.rect.y += vy * timePassed;
    }

    public void moveRight(long timePassed)
    {
        xPos += vx * timePassed;
        this.rect.x += vx * timePassed;
    }

    public void moveLeft(long timePassed)
    {
        xPos -= vx * timePassed;
        this.rect.x -= vx * timePassed;
    }


//I passed timePassed through a globalInputObject because my input is handled in public //void keyPressed(KeyEvent e) and I didnt know how else to get timePassed in to the //movement method

//Here is the code in gameLoop()
                 globalInputObject.isPacManMovingUp(timePassed);
             globalInputObject.isPacManMovingDown(timePassed);
             globalInputObject.isPacManMovingRight(timePassed);
         globalInputObject.isPacManMovingLeft(timePassed);




//This is inside the GlobalInputObject
public void isPacManMovingUp(long timePassed)
    {
        if(pacMan.movingUp == true)
        {
            pacMan.moveUp(timePassed);
        }
    }

    public void isPacManMovingDown(long timePassed)
    {
        if(pacMan.movingDown == true)
        {
            pacMan.moveDown(timePassed);
        }
    }

    public void isPacManMovingRight(long timePassed)
    {
        if(pacMan.movingRight == true)
        {
            pacMan.moveRight(timePassed);
        }
    }

    public void isPacManMovingLeft(long timePassed)
    {
        if(pacMan.movingLeft == true)
        {
            pacMan.moveLeft(timePassed);
        }
    }

Answer 1

Rather than always moving the pacman by a constant distance (1 pixel, it appears) each time the timer runs, you should:

1. Set the timer to run as fast as possible (e.g. once every millisecond or less). Edit: if you set it too fast, the game may end up actually running slower, you'll have to experiment.
2. Calculate how much time has passed between each frame using the system clock and move the pacman by an amount proportional to that.

Doing the above will mean that if the system is "laggy," it will simply show fewer frames per second, rather than actually moving everything slower.