以下のプログラムで、マップ上を横にキャラクタを動かそうとしています。しかしながら、明らかに画面の更新が遅く、ちらつきがみえます。なにがマズイのでしょうか？一つお願いします。

// ƒ}ƒbƒv•\Ž¦Šî–{
#include "DxLib.h"
#include <stdio.h>
#include <string.h>
#include <fstream>
#include <iomanip>
#include <iostream>
using namespace std;

#define MAP_SIZE 32			// &#402;}&#402;b&#402;v&#402;`&#402;b&#402;v&#710;&ecirc;&#8218;&Acirc;&#8218;&Igrave;&#402;h&#402;b&#402;g&#402;T&#402;C&#402;Y

#define MAP_WIDTH 20			// &#402;}&#402;b&#402;v&#8218;&Igrave;&#8226;&#157;
#define MAP_HEIGHT 16			// &#402;}&#402;b&#402;v&#8218;&Igrave;&#143;c’&middot;&#8218;&sup3;

void DrawMap();
void GraphDraw(int image[]);

// &#402;}&#402;b&#402;v&#8218;&Igrave;&#402;f&#129;[&#402;^
int MapData[ MAP_HEIGHT ][ MAP_WIDTH ];


int WINAPI WinMain( HINSTANCE hInstance, HINSTANCE hPrevInstance,
						 LPSTR lpCmdLine, int nCmdShow )
{
//	ChangeWindowMode( TRUE ) ;
     
        char Key[256];

	if( DxLib_Init() == -1 )	// &#8218;c&#8218;w&#402;‰&#402;C&#402;u&#402;‰&#402;&#352;&#143;‰&#352;&uacute;‰&raquo;&#143;&#710;&#8212;&#157;
	{
		 return -1;		// &#402;G&#402;‰&#129;[&#8218;&ordf;&#8249;N&#8218;&laquo;&#8218;&frac12;&#8218;&ccedil;’&frac14;&#8218;&iquest;&#8218;&Eacute;&#143;I&#8212;&sup1;
	}
	       
        SetDrawScreen(DX_SCREEN_BACK);

	SetGraphMode( 640 , 480 , 16 ) ;
	 DrawMap();
        int image[480],x=640/2,y=480/2,GMima;
	LoadDivGraph("dang01_a.bmp",480,30,16,32,32,image);
	GMima = LoadGraph("GM.bmp");
	

	
	
	while(1){
		ClearDrawScreen();   //&#8212;&nbsp;‰&aelig;&#8211;&Ecirc;&#8218;&Igrave;&#402;f&#129;[&#402;^&#8218;&eth;‘S&#8218;&Auml;&#141;&iacute;&#143;&#339;
            	GetHitKeyStateAll( Key ) ; 
	


		if( Key[ KEY_INPUT_RIGHT ]  == 1 )                     //‰E&#402;{&#402;^&#402;“&#8218;&ordf;‰&#376;&#8218;&sup3;&#8218;&ecirc;&#8218;&frac12;&#8218;&ccedil;
                	x++ ;                                              //x&#8218;&Igrave;’l&#8218;&eth;&#8218;P‘&#157;&#8218;&acirc;&#8218;&middot;

            	else if(x%32!=0)
                	x++;
			
		if( Key[ KEY_INPUT_ESCAPE ]  == 1 ) break ;
		if( ProcessMessage() == -1 ) break ; 
                GraphDraw(image);
		DrawGraph(x,y,GMima,FALSE);	
		
		ScreenFlip(); 

        }


	DxLib_End() ;		// &#8218;c&#8218;w&#402;‰&#402;C&#402;u&#402;‰&#402;&#352;&#381;g&#8212;p&#8218;&Igrave;&#143;I&#8212;&sup1;&#143;&#710;&#8212;&#157;

	return 0 ;		// &#402;\&#402;t&#402;g&#8218;&Igrave;&#143;I&#8212;&sup1;
}

void DrawMap(){
	int i=0,j=0,temp; 
	ifstream cFStrm( "Data.txt");   

        while( cFStrm >> temp ){
		MapData[i][j] = temp;
		if(j==MAP_WIDTH-1){
			i++;
			j = 0;
		}
	        else{
			j++;
		}
	} 
      	
	cFStrm.close();

}

void GraphDraw(int image[]){
	int i , j ;
		// &#402;}&#402;b&#402;v&#8218;&eth;&#8226;`&#8218;&shy;
		for( i = 0 ; i < MAP_HEIGHT ; i ++ )
		{
			for( j = 0 ; j < MAP_WIDTH ; j ++ )
			{
				if( MapData[ i ][ j ] == 0 )
				{
					DrawGraph(j*MAP_SIZE,i*MAP_SIZE,image[37],TRUE);
				}
				else if(MapData[i][j]==1){
					DrawGraph(j*MAP_SIZE,i*MAP_SIZE,image[34],TRUE);
				}
				else if(MapData[i][j]==2){
					DrawGraph(j*MAP_SIZE,i*MAP_SIZE,image[35],TRUE);
				}
				else{
					DrawGraph(j*MAP_SIZE,i*MAP_SIZE,image[36],TRUE);
				}
			}
		}


}

初心なもので、汚いソースについてはご容赦を。