PHP hangs up with ImageMagik

[cpegphp]

This is the part of my script where i call imagemagik. everything works fine, but after 18 runs it hangs up.

System: Win2000, PHP 4.1.1, Apache 1.3.2

The Apache.exe, cmd.exe, php.exe and mogrify.exe hang up and i can't kill them.

What can i do?

Script:

copy ( $picture , "$server_root$viewpath/$thumb_pic_name/t_$bild_name[$i]");

$start_this = "$server_root/scripts/ImageMagick/mogrify.exe -dither -geometry $width_get x $hight_get ! -normalize $server_root$viewpath/$thumb_pic_name/t_$bild_name[$i] > nul";

exec ( $start_this );