var iAantalVakken = 4;
		var sBefore = "part";
		var sShown = "part001";
		var iTimer = 3000;
	
		function showRandomBlok()
		{
			var iRandom = Math.random();
			var iShow = Math.round( iRandom * (iAantalVakken - 1) ) + 1;
			var sShow = sBefore + PadDigits( iShow, 3 );
			document.getElementById( sShown ).style.display = 'none';
			document.getElementById( sShow ).style.display = 'block';
			sShown = sShow;
			setTimeout( 'showRandomBlok()', iTimer );
		}
		
		function PadDigits(n, totalDigits) 
    { 
			n = n.toString(); 
			var pd = ''; 
			if (totalDigits > n.length) 
			{ 
				for (i=0; i < (totalDigits-n.length); i++) 
				{ 
					pd += '0'; 
				} 
			} 
			return pd + n.toString(); 
    }