Graphics Lag on Certain Computers

Hi, I've been learning how to program in Java for a while now, and made a small game to see what I could implement (Here's the compiled version: Burger)
I'm obviously doing something wrong with the screen refreshing, and I need a more efficient method. I made a separate thread that repaints the screen, and it sleeps for 3 miliseconds. If I make it sleep more, the game is very slow (the guy moves one pixel at a time, by the way).
On certain computers, the game runs fine (you can get 40 + points). On others, you can barely get a piece of food before dying. Can anyone help me with this? I've heard of buffered images (I just use images) , but I have no idea how to use them.
Also, I wouldn't mind receiving some feedback on the game and what I should add

I have to say, I really like the concept of your game.

It runs smoothly for me, maybe you can post your code and we can see where you can make some improvements.

Here is the main part of the code (although I cut a lot)

package com.jctrl;

import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import java.net.URL;
import java.io.*;
import java.awt.image.*;
import java.util.Timer;
import java.util.TimerTask;

public class Burger{
	
	public static void main(String[] args){
		Burger game = new Burger();
		game.build();
	}

Stick man = new Stick();
	Mover moving = new Mover(this);

DPanel panel;
	JFrame frame;

public void build(){

panel = new DPanel();
		frame = new JFrame("Burger");
		
		frame.setBackground(Color.WHITE);
		frame.setContentPane(panel);	
		frame.addKeyListener(moving);
		frame.setSize(500, 500);
		frame.setLocationRelativeTo(null);
		frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
		frame.setVisible(true);
		frame.setResizable(false);

man.init();
		
		Runnable painterRun = new Painter();
		Thread paintThread = new Thread(painterRun);
		paintThread.start();
	}			
	class DPanel extends JPanel{

public void paintComponent(Graphics g) {

Graphics2D g2 = (Graphics2D) g;
			
			movement();

g.setColor(Color.WHITE);
			g.fillRect(0, 0, 500, 25);
			g.fillRect(man.guy.x, man.guy.y, man.guy.width, man.guy.height);
			g.setColor(Color.GREEN);
			g.fillRoundRect(85, 2, 320, 20, 3, 10);
			g.setColor(Color.WHITE);
			g.fillRect(405 - bar, 2, 320, 20);
			g.setColor(Color.BLACK);		
			g.drawRoundRect(85, 2, 320, 20, 3, 10);
			g.setFont(font);
			g.drawString(Integer.toString(score), 2, 20);

if (size.equals("normal")){
				g.setColor(Color.BLUE);

}else if (size.equals("trim")||size.equals("chubby")){
				g.setColor(Color.GREEN);

}else if (size.equals("fat")||size.equals("slim")){
				g.setColor(Color.ORANGE);

}else if (size.equals("obese")||size.equals("dying")){
				g.setColor(Color.RED);

}else{
				g.setColor(Color.BLACK);

}
			g.drawString(size, 420, 20);

for (int x = 0; x < junk.length; x++){
				g.drawImage(junk[x].foodImage, junk[x].food.x, junk[x].food.y, this);
			}
			g.drawImage(man.guyImage, man.guy.x, man.guy.y, this);
			
			g.setColor(Color.BLACK);
	
		}
	}

public class Painter implements Runnable{
		public void run(){
		
			while(true){
				panel.repaint();
				try{
					Thread.sleep(3);
				}catch(Exception ex){}
			}
		}
	}

public void movement(){

if(end == false){
			for (int x = 0; x < 4; x++){
				if (moving.getKeyPress(x)){
					man.move(x);
				}
			}
		}
	}

}

I also have a class called Stick, which is basically the character. This class loads the images and handles which one to use (I cut some stuff here too)

public class Stick{

//sizes: Dying, slim, trim, normal, chubby, fat and obese

Image dying = Toolkit.getDefaultToolkit().getImage(Burger.class.getResource("guy1.gif"));
	Image slim = Toolkit.getDefaultToolkit().getImage(Burger.class.getResource("guy2.gif"));
	Image trim = Toolkit.getDefaultToolkit().getImage(Burger.class.getResource("guy3.gif"));
	Image normal = Toolkit.getDefaultToolkit().getImage(Burger.class.getResource("guy4.gif"));
	Image chubby = Toolkit.getDefaultToolkit().getImage(Burger.class.getResource("guy5.gif"));
	Image fat = Toolkit.getDefaultToolkit().getImage(Burger.class.getResource("guy6.gif"));
	Image obese = Toolkit.getDefaultToolkit().getImage(Burger.class.getResource("guy7.gif"));
	Image dead = Toolkit.getDefaultToolkit().getImage(Burger.class.getResource("guy8.gif"));

Image guyImage = normal;
	Rectangle guy = new Rectangle(245, 235, 36, 57);
	int currentSize = 3;
	String returnSize;

public void move(int key){

if(key == 3){
				guy.x += 1;

}else if (key == 2){
				guy.x -= 1;
			}
			
			if (key == 1 == true){
				guy.y += 1;

}else if (key == 0){
				guy.y -= 1;
			}
	}
}

And this is the Mover class. It's the Action Listener for the game. I have a boolean array that represents the arrow keys. If one is pressed, it becomes true and all the others become false.
Again, it's incomplete.

package com.jctrl;

import java.awt.*;
import java.awt.event.*;

public class Mover implements KeyListener{
	
	Burger boss;

public Mover(Burger temp){
		boss = temp;
	}

public Mover(){
	}

boolean[] keyPress = new boolean[4];

public void keyPressed(KeyEvent input){

int key = input.getKeyCode();

if(key == KeyEvent.VK_RIGHT){
			keyPress[3] = true;
			keyPress[2] = false;
			keyPress[1] = false;
			keyPress[0] = false;

} else if(key == KeyEvent.VK_LEFT){
			keyPress[2] = true;
			keyPress[1] = false;
			keyPress[3] = false;
			keyPress[0] = false;

} else if(key == KeyEvent.VK_DOWN){
			keyPress[1] = true;
			keyPress[2] = false;
			keyPress[3] = false;
			keyPress[0] = false;

} else if(key == KeyEvent.VK_UP){
			keyPress[0] = true;
			keyPress[2] = false;
			keyPress[3] = false;
			keyPress[1] = false;
		}

}

public void keyTyped(KeyEvent input) {}
			
		
	public void keyReleased(KeyEvent input){}
	
	public boolean getKeyPress(int key){

return keyPress[key];
	}

}

I also have a Meal class, which handles the food that appears on-screen. Since I'm having problems with the graphics, I didn't post that.
I hope this is enough to find a solution.