极度郁闷，请高人指教线程问题

实例Runnable接口，在Applet上实现：每次比前一次多一笔的简单动画效果。部分代码如下：
public void run(){
Thread.currentThread().setPriority(Thread.MIN_PRIORITY);
while(Thread.currentThread()==thread){
repaint();
flag++;
try{
    Thread.sleep(1000);
}catch(InterruptedException e){
break;
}
}
} public void paint(Graphics gg){
g.setColor(Color.black);
switch(flag){
case 1: 

g.drawLine(X+80,Y+25,X+80,Y+25+35);
         break;
case 2:
g.drawLine(X+80,Y+25+60,X+80,Y+25+35+60);

break;
case 3: 
g.drawLine(X+80,Y+25+60+60,X+80,Y+25+35+60+60);
break;
case 4: 
g.drawLine(X+80,Y+25+35+60+60+60,X+80+5,Y+25+35-10+60+60+60);
break;
case 5:
g.drawLine(X+80,Y+25+35+60+60+60+60,X+80+5,Y+25+35-10+60+60+60+60);
break;
case 6:
g.drawLine(X+80,Y+25+35+60+60+60+60+60,X+80+5,Y+25+35-10+60+60+60+60+60);
break;
case 7:
            g.drawLine(X+160,Y+25+60+60+60+5,X+160-3,Y+25+60+60+5-3);

break;
case 8:
g.drawLine(X+220,Y+25+60+60+60-60,X+220+5,Y+25+60+60+60+5);

break;
case 9:

g.drawLine(X+220,Y+25++60-60-60,X+220-5,Y+25-60+5);
          break;
                   }
为什么有时运行成功，有时中间少了一笔或几笔？？？？？？？？？