There is a bug in PHP that is plaguing many developers and site owners right now which pertains to processing a 'large' string with preg_replace. Something to do with the apache stack.
Long story short: this line (improved-simpler-css.php:251) brings my site down
$_return = preg_replace ( '/@import.+;( |)|((?:\/\*(?:[^*]|(?:\*+[^*\/]))*\*+\/))/i', '', $_content );
if I replace with
$_return = $_content everything is fine. I'm OK with the extra load time for now. I'm probably going to try splitting the replace statement into multiple steps.
more info/examples: http://www.apachefriends.org/f/viewtopic.php?f=16&t=45367#p184056