GLSL + 6800 + Multiple steps by frame

Hello Everybody

I am programming with 3D textures and GLSL. At some point, I was interested 
in making 2 renderings in one frame, for comparrison...

void DrawScene()
{
  glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
  Render();
  glFlush();
  ReadFrameBuffer(A);
  glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
  Render();
  glFlush();
  ReadFrameBuffer(B);
  difference = B-A;
  // Do something with difference, here!....
  glFlnish();
  glutSwapBuffer();
}

The second rendering is not correct... I get a mess. But, If I put an 
Sleep(200) after every glFluch, everything is ok!.... Coudl you give me any 
clue about what the hell is goiing on?

Cheers Guys!

Rix

Note: I am using latest drivers.... 

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