OpenGL flicker only on Windows 8 - opengl

Question

I have a Windows application which uses OpenGL in a fairly simple way to render textured images (QUADS) into multiple windows in its client-area.
The application behaves fine on Windows 7, but on Windows 8 it sometimes flickers and shows the background in response to user actions, like a mouse click on a window. To be exact: sometimes when some window is mouse-clicked (not on every mouse click), all windows briefly flicker.
I was looking into possible reasons for this behavior. I tried catching the WM_ERASEBKGND message, but that didn't seem to be the cause. I tried working with different GPU cards (and corresponding drivers), and it happens on all (Nvidia Quadro & ATI Radeon HD). In addition, I made some of the image-windows use GDI for drawing, and indeed - when other windows flicker, these images don't (as well as the menu, button area & status bar).
UPDATE: Since the windows are C# User Controls, I've set the AllPaintingInWmPaint & UserPaint Styles to "true". DoubleBuffer is "false", because setting it "true" causes the flicker to be a lot more frequent.
Bottom line: all windows drawn with OpenGL flicker once in a while, and only them, and only on Win8 (not on Win7).
I would very much appreciate tips for the cause of this. My code follows. I initialize the contexts like this:
void InitWindow(HWND *hWnd, HDC *hDC, HGLRC *hRC)
{
*hDC = GetDC(*hWnd);
PIXELFORMATDESCRIPTOR pfd;
ZeroMemory(&pfd, sizeof(pfd));
pfd.nSize = sizeof(pfd);
pfd.nVersion = 1;
pfd.dwFlags = PFD_DRAW_TO_WINDOW | PFD_SUPPORT_OPENGL |
PFD_DOUBLEBUFFER;
pfd.iPixelType = PFD_TYPE_RGBA;
pfd.cColorBits = 24;
// pfd.cRedBits = 8;
// pfd.cGreenBits = 8;
// pfd.cBlueBits = 8;
pfd.cAlphaBits = 0;
pfd.cDepthBits = 0;
pfd.iLayerType = PFD_MAIN_PLANE;
int format = ChoosePixelFormat(*hDC, &pfd);
SetPixelFormat(*hDC, format, &pfd);
*hRC = wglCreateContext(*hDC);
wglMakeCurrent(*hDC, *hRC);
}
And draw to the screen like this:
int Render(HWND hWnd, Raster* raster)
{
Size RasterSize = raster->GetSize();
HDC hDC = NULL;
HGLRC hRC = NULL;
InitWindow(&hWnd, &hDC, &hRC);
GLenum raster_pixel_format = GL_BGRA_EXT;
GLint internal_format = GL_RGBA;
GLuint texture;
glGenTextures(1, &texture);
glBindTexture (GL_TEXTURE_2D, texture);
glPixelStorei (GL_UNPACK_ALIGNMENT, 1);
glTexParameteri (GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP);
glTexParameteri (GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP);
glTexParameteri (GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
glTexParameteri (GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
glTexEnvf (GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_MODULATE);
glTexImage2D (GL_TEXTURE_2D, 0, internal_format,
RasterSize.width, RasterSize.height,
0, raster_pixel_format, GL_UNSIGNED_BYTE,
raster->GetData());
glBindTexture(GL_TEXTURE_2D, texture);
RECT wndRect;
::GetClientRect(hWnd, &wndRect);
GLsizei wndWidth = wndRect.right;
GLsizei wndHeight = wndRect.bottom;
glEnable(GL_TEXTURE_2D);
// this is usually stated in window coordinates,
// but since we know the raster gets its size from
// the window - we can use raster coordinates
glViewport(0, 0, RasterSize.width, RasterSize.height);
glBegin( GL_QUADS );
glTexCoord2d(0.0,0.0); glVertex2d(-1.0,+1.0);
glTexCoord2d(1.0,0.0); glVertex2d(+1.0,+1.0);
glTexCoord2d(1.0,1.0); glVertex2d(+1.0,-1.0);
glTexCoord2d(0.0,1.0); glVertex2d(-1.0,-1.0);
glEnd();
glDisable(GL_TEXTURE_2D);
glBindTexture(GL_TEXTURE_2D, 0);
glDeleteTextures(1, &texture);
SwapBuffers(hDC);
wglMakeCurrent(NULL, NULL);
wglDeleteContext(hRC);
ReleaseDC(hWnd, hDC);
return SUCCESS;
}

Answer 1

Issue was due to using a machine with multiple video cards.
Disabling any one card solved it.