I have a sketch stretching a bunch of colored vertical lines by a sinewave, however there is a weird flicker when I run it, as if the bottom of the sketch is getting updated after the top.  I'd really like to know what's at play here.  I tried buffering and that didn't seem to help, so if anyone can help me I'd appreciate it.  Thanks.

void setup() {
 size(800,800);
 
 colors = new int[1500];
 int j = 0;
 for (int i = 0; i<1500;i++) {
   if (random(1) < .5) {
     colors[i]= color((int)random(255)+000, (int)random(255)+000,(int)random(2550)+000);
   }
   else {
     colors[i]= color((int)random(2550)+000, (int)random(255)+000,(int)random(255)+000);      
   }
 }

 x = 0;
 
 frameRate(20);
 
}
color colors[];
int sc = 10;
int top = 600;
int bend = 000;
int jj=0;
void draw() {

 if (!tbool) {
    jj+=1;

   if (random(100) > 50) {
     bend++;
   }
   else {
     bend--;
   }

   if (random(100) < 5) {
     top+=5;
   }
   else {
     top-=5;
   }
   int j = jj;
   for (int i = 0; i<width+100;i+=abs(sin(x))*sc) {

     if (j >= 1500) {
      j = 0;
     }
     stroke(colors[j++]);      

     strokeWeight(abs(sin(x))*sc+1);
     
     x+=1.0/10;
     if (x > PI) {
       x = 0;
     }
     line(i, height, i, bend);
   }
}
 
}
float x;
boolean tbool;
void mousePressed() {
 tbool = !tbool;
}

it could be your vertical sync on the graphics card needs switched on.