style sheet for sub-theme not being applied

Hi all,

I created a sub-theme using the Zen theme's starter kit. About every 40 or so clicks navigating around my site where my style sheet comes up properly - all of a sudden it isn't called and none of my style sheet's attributes come up at all. I get all the content on the page as usual, just no styles applied except the Drupal core module style sheets. I'm trying to figure out a way for certain that my custom css file gets called every time. I've also tried putting <link type="text/css" rel="stylesheet" media="all" href="http://www.example.com/sites/all/themes/zen/custom_theme/custom_theme.css" /> and a

The style sheet disappears very often when using FireFox on a Mac computer, but I have seen it happen occasionally on IE for Windows too. It hasn't been a problem on Safari on a Mac computer.
I'm using Drupal 6 and a sub-theme I created from Zen theme's starter kit. Has anyone else had this issue before? Any possible solutions come to mind?

Thanks,
Andrew G