设置的消息钩子会失效！

[shan794462554]

static HWND  hwnd;  		// MainForm句柄
static HHOOK hKeyHook; 	    // HOOK句柄

CKeyHook::CKeyHook(HWND hWnd)
{
	hwnd = hWnd;
	hKeyHook   = SetWindowsHookEx( WH_KEYBOARD_LL,(HOOKPROC)_HookProc,GetModuleHandle(NULL),0);
}

//----------------------------------------------------------------------------------------------------
LRESULT CALLBACK CKeyHook::_HookProc(int code,WPARAM wParam,LPARAM lParam)
{
PKBDLLHOOKSTRUCT pKey ;

		if((code==HC_ACTION) )
		{
			pKey = (PKBDLLHOOKSTRUCT)(lParam);
			if((wParam==WM_KEYDOWN))
			{
//				pKey = (PKBDLLHOOKSTRUCT)(lParam);
				if(VK_ESCAPE == pKey->vkCode )
				{
					PostMessage( hwnd,MYESC,wParam,pKey->vkCode);
					return CallNextHookEx(hKeyHook,code,wParam,lParam);
				}

				//SHIFT + F12
				if((GetKeyState(VK_CONTROL) < 0) && (GetKeyState(VK_MENU) < 0))
				{
//					pKey = (PKBDLLHOOKSTRUCT)(lParam);
					PostMessage( hwnd,OTHER_BORD,wParam,pKey->vkCode);
					return CallNextHookEx(hKeyHook,code,wParam,lParam);
				}
				//SHIFT + SPACE
				if((GetKeyState(VK_SHIFT) < 0))
				{
//					pKey = (PKBDLLHOOKSTRUCT)(lParam);
					PostMessage( hwnd,INSERT_SPACE,wParam,pKey->vkCode);
					return CallNextHookEx(hKeyHook,code,wParam,lParam);
				}
			}
			else if((wParam==WM_KEYUP))
			{
				if(VK_F6 == pKey->vkCode )
				{
					PostMessage( hwnd,CUTIMAGE,wParam,pKey->vkCode);

					return CallNextHookEx(hKeyHook,code,wParam,lParam);
				}
			}
		}

	return CallNextHookEx(hKeyHook,code,wParam,lParam);
}

用消息钩子实现快捷键F6并对所有界面都有效，现在在调试阶段按F6键2到3次后消息钩子就失效了，这怎么解决？