GetDC(HWND_DESKTOP) stops after 9997 calls

The code below writes in console x,y and color of a pixel pointed by the mouse. 
It's working, but has 3 faults:

1. After 9997 calls GetDC(HWND_DESKTOP) stops giving sensible data.
My pragmatic solution is to count loops, call a link pointing to exe and exit.
2. Size of memory used increases - of order of 1 kB / loop.
3. It is slow - 3 milliseconds is a high price for checking color of a single pixel.

Please tell me how to correct those problems, especially 1 and 3.
I mean something simple and usable with MS Express Edition 2013 and Windows 7.
I use similar code to simulate response of a user to somebody else's application by monitoring color of a few points on the screen.

#include "stdafx.h"
#include "iostream"
#include "mmdeviceapi.h"

using namespace std;

int _tmain(int argc, _TCHAR* argv[])
{
  int x, y, n = 0;	HDC hdc;	POINT curPos;	BOOL result;
    do {
      hdc = GetDC(HWND_DESKTOP);		n++;
      if (n > 9990) { 
        ShellExecute(NULL, L"open", L"\"C:\\Users\\me\\Desktop\\xyColor.lnk\"", 
	             NULL, NULL, SW_SHOWDEFAULT);
         exit(0);
    } 
    result = GetCursorPos(&curPos);
    if (result) {
      x = curPos.x;
      y = curPos.y;
      cout << "n=" << n << " x=" << x << " y=" << y 
           << hex << " BGR= " << GetPixel(hdc, x, y) << dec << endl;
      Sleep(10);
    }
  } while (result);
  return 0;
}