DirextDraw双缓冲问题，出现重叠

Question

明明清除了视频内存中的数据，Flip绘制时为何绘制的时候出现重叠，当设置位置为（600，400）时，如绘制的图像FPS=1 与FPS=2重叠...后续的绘制不断重叠，如果绘制的坐标（0，400）较小时不会出现重叠现象，求解答！

 在这里书写代码片  
 while(TRUE)///主循环
    {
        if(::PeekMessage(&msg,NULL,0 ,0 ,PM_REMOVE))
        {
            if(msg.message  ==  WM_QUIT)
                break;
            TranslateMessage(&msg);
            DispatchMessage(&msg);
        }
        //////////////the game processing goes here 
        else
        {
            static char info[10];
            memset(info,0,10);
            sprintf(info,"FPS=%0.0f",++Fps);
            CGAME::Game_Main(info,1);//绘制图像
        }
    }

    ///////CGAME::Game_Main函数
int CGAME::Game_Main(void* parms,int num_parms )
{
    CDX_::lpddsd    =   new DDSURFACEDESC2();
    DDRAW_INIT_STRUCT(CDX_::lpddsd);
    CDX_::lpddsback->Lock(NULL,CDX_::lpddsd,
                                        DDLOCK_SURFACEMEMORYPTR|DDLOCK_WAIT,
                                        NULL);
    UCHAR*  back_buffer = (UCHAR*)CDX_::lpddsd->lpSurface;
    //////清除了内存
    if(CDX_::lpddsd->lPitch ==  SCREEN_WIDTH)
        memset(back_buffer , 0 ,SCREEN_WIDTH*SCREEN_HEIGHT);
    else
    {
        UCHAR   *dest_ptr  = back_buffer;
        for(int y =0 ; y<SCREEN_HEIGHT;++y)
        {
            memset(dest_ptr ,0 ,SCREEN_WIDTH);
            dest_ptr+=CDX_::lpddsd->lPitch;         
        }
    }///////填充backsurface
    CDX_::lpddsback->Unlock(NULL);
    ///////绘制GDI，绘制的坐标为（600，400）
    CGAME::GDI_DRAW((char*)parms,600,400,
        RGB(rand()%255,rand()%255,rand()%255),CDX_::lpddsback);
    while(FAILED(CDX_::lpddsprimary->Flip(NULL,DDFLIP_WAIT)));
    Sleep(200);
    delete      CDX_::lpddsd;
    CDX_::lpddsd    =   NULL;
    return 1;
}

//////////绘制CGAME::GDI_DRAW使用GDI绘制

 int    CGAME::GDI_DRAW(char *text, int x,int y,
                                              COLORREF color, LPDIRECTDRAWSURFACE7 lpdds)
{
        HDC xdc; 
        if (FAILED(lpdds->GetDC(&xdc)))
           return(0);
        SetTextColor(xdc,color);
        SetBkMode(xdc, TRANSPARENT);
        TextOut(xdc,x,y,text,strlen(text));
        lpdds->ReleaseDC(xdc);
        return(1);
}