IIS 7 mit PHP - Download bricht immer bei 81,8MB ab

function dos_filesize($fn) {
    if (is_file($fn))
        return exec('FOR %A IN ("'.$fn.'") DO @ECHO %~zA');  
    else
        return '0';  
}


if (is_file($file['path'])) {  

	//	$length = $file['size'] = shell_exec('ls -l "'.$file['path'].'" | cut -f 5 -d " "');	# LINUX 
		$length = $file['size'] = dos_filesize($file['path']);									# WIN  
		$f = fopen($file['path'], 'rb');  
		$start  = 0;
		if (preg_match('/^bytes=(\d*)-(\d*)$/i', $config['range'], $r)) {  
				if ($r[1] == '')   
					$start = 0; 
				else 
					$start = min($r[1], $file['size'] - 1);  
				if ($r[2] == '')   
					$length = $file['size'] - $start; 		  
				else
					$length = min($r[2], $file['size'] - 1) - $start + 1;  
					
				header('HTTP/1.1 206 Partial content');  
				header('Content-Range: bytes '.$start.'-'.($start + $length - 1).'/'.$file['size']);  
				
		}
		header('Pragma: no-cache');  
		header('Expires: 0');  
		header('Cache-Control: private');  
		header('Cache-Control: no-store, no-cache, must-revalidate');  
		header('Cache-Control: post-check=0, pre-check=0, false');  
		header('Accept-Ranges: bytes');			  
		header('Content-Type: application/octet-stream; charset=utf-8');   
		header('Content-Disposition: attachment; filename="'.$file['name'].'"');   
		header('Content-Length: '.$length);	  
		header('Content-Transfer-Encoding: binary');  
		
		if ($config['maxspeed'] > 0)  
			$buffsize = $config['maxspeed'];  
		else
			$buffsize = 4096;
			
		$read = 0;
		fseek($f, $start);
		while (!feof($f) && ($read < $length)) {
			$buff = fgets($f, min($buffsize, $length - $read));
			$read += strlen($buff);
			echo $buff;
			if ($config['maxspeed'] > 0) {  
				flush();
				sleep(1);
			}
		}
		fclose($f);
}