GD Library work unstable when I try to increase picture dimentions. For example 
simple script:

<?php Header("Content-type: image/gif");
$im_width = 128; 
$im_heigth = 32;

$im_gif = imagecreate($im_width, $im_heigth);

$color_white = imagecolorallocate($im_gif, 255, 255, 255);
$color_black = imagecolorallocate($im_gif, 0, 0, 0);

imagefill($im_gif, 0, 0, $color_white);
imagestring($im_gif, 2, 4, 4, "QWERTY", $color_black); 
imagegif($im_gif);
?>

- still work, but if 

$im_width = 256; 
$im_heigth = 256;

- then only half of image displayed, then became noise :)
It's look like wrong memory allocation for array of pixels (or limit in 16Kb -???).

P.S. I, of carse, read in read.me file that GD library is still unstable, but possibly it's my mistakes? 
Or not?
P.P.S. I use PHP3.0beta6 on Windows NT Server with IIS3.
GD Library activated in "ini" file: (extension=gd.dll)